Repeating digit in an infinite decimal. Infinite periodic fractions

The fact that many square roots are irrational numbers, does not detract from their significance, in particular, the number $\sqrt2$ is very often used in various engineering and scientific calculations. This number can be calculated with the accuracy that is necessary in each specific case. You can get this number with as many decimal places as you have the patience for.

For example, the number $\sqrt2$ can be determined to six decimal places: $\sqrt2=1.414214$. This value is not very different from the true value, since $1.414214 \times 1.414214=2.000001237796$. This answer differs from 2 by just over one millionth. Therefore, the value of $\sqrt2$, equal to $1.414214$, is considered quite acceptable for solving most practical problems. In the case when greater precision is required, it is not difficult to obtain as many significant digits after the decimal point as necessary in this case.

However, if you show rare stubbornness and try to extract Square root from the number $\sqrt2$ until you achieve the exact result, you will never finish your work. It's an endless process. No matter how many decimal places you get, there will always be a few more.

This fact can amaze you as much as turning $\frac13$ into an infinite decimal $0.333333333…$ and so on infinitely or turning $\frac17$ into $0.142857142857142857…$ and so on infinitely. At first glance, it may seem that these infinite and irrational square roots are phenomena of the same order, but this is not at all the case. After all, these infinite fractions have a fractional equivalent, while $\sqrt2$ has no such equivalent. And why, exactly? The fact is that the decimal equivalent of $\frac13$ and $\frac17$, as well as an infinite number of other fractions, are periodic infinite fractions.

At the same time, the decimal equivalent of $\sqrt2$ is a non-periodic fraction. This statement is also true for any irrational number.

The problem is that any decimal that is an approximation of the square root of 2 is non-periodic fraction. No matter how far we advance in the calculations, any fraction we get will be non-periodic.

Imagine a fraction with a huge number of non-periodic digits after the decimal point. If suddenly after the millionth digit the whole sequence of decimal places is repeated, then decimal- periodic and for it there is an equivalent in the form of a ratio of integers. If a fraction with a huge number (billions or millions) of non-periodic decimal places at some point has an endless series of repeating digits, for example $…55555555555…$, this also means that this fraction is periodic and there is an equivalent for it in the form of a ratio of integers numbers.

However, in the case of their decimal equivalents are completely non-periodic and cannot become periodic.

Of course, you can ask the following question: “And who can know and say for sure what happens to a fraction, say, after a trillion sign? Who can guarantee that the fraction will not become periodic? There are ways to irrefutably prove that irrational numbers are non-periodic, but such proofs require complex mathematical apparatus. But if it suddenly turned out that an irrational number becomes periodic fraction, this would mean a complete collapse of the foundations of the mathematical sciences. And in fact, this is hardly possible. This is not just for you to throw on the knuckles from side to side, there is a complex mathematical theory here.

This article is about decimals. Here we will deal with the decimal notation of fractional numbers, introduce the concept of a decimal fraction and give examples of decimal fractions. Next, let's talk about the digits of decimal fractions, give the names of the digits. After that, we will focus on infinite decimal fractions, say about periodic and non-periodic fractions. Next, we list the main actions with decimal fractions. In conclusion, we establish the position of decimal fractions on the coordinate ray.

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Decimal notation of a fractional number

Reading decimals

Let's say a few words about the rules for reading decimal fractions.

Decimal fractions, which correspond to the correct ordinary fractions, are read in the same way as these ordinary fractions, only “zero whole” is added beforehand. For example, the decimal fraction 0.12 corresponds to the ordinary fraction 12/100 (it reads “twelve hundredths”), therefore, 0.12 is read as “zero point twelve hundredths”.

Decimal fractions, which correspond to mixed numbers, are read in exactly the same way as these mixed numbers. For example, the decimal fraction 56.002 corresponds to a mixed number, therefore, the decimal fraction 56.002 is read as "fifty-six point two thousandths."

Places in decimals

In the notation of decimal fractions, as well as in the notation of natural numbers, the value of each digit depends on its position. Indeed, the number 3 in decimal 0.3 means three tenths, in decimal 0.0003 - three ten thousandths, and in decimal 30,000.152 - three tens of thousands. Thus, we can talk about digits in decimals, as well as about digits in natural numbers.

The names of the digits in the decimal fraction to the decimal point completely coincide with the names of the digits in natural numbers. And the names of the digits in the decimal fraction after the decimal point are visible from the following table.

For example, in the decimal fraction 37.051, the number 3 is in the tens place, 7 is in the units place, 0 is in the tenth place, 5 is in the hundredth place, 1 is in the thousandth place.

The digits in the decimal fraction also differ in seniority. If we move from digit to digit from left to right in the decimal notation, then we will move from senior to junior ranks. For example, the hundreds digit is older than the tenths digit, and the millionths digit is younger than the hundredths digit. In this final decimal fraction, we can talk about the most significant and least significant digits. For example, in decimal 604.9387 senior (highest) the digit is the hundreds digit, and junior (lowest)- ten-thousandth place.

For decimal fractions, expansion into digits takes place. It is analogous to the expansion in digits of natural numbers. For example, the decimal expansion of 45.6072 is: 45.6072=40+5+0.6+0.007+0.0002 . And the properties of addition from the expansion of a decimal fraction into digits allow you to go to other representations of this decimal fraction, for example, 45.6072=45+0.6072 , or 45.6072=40.6+5.007+0.0002 , or 45.6072= 45.0072+0.6 .

End decimals

Up to this point, we have only talked about decimal fractions, in the record of which there is a finite number of digits after the decimal point. Such fractions are called final decimal fractions.

Definition.

End decimals- These are decimal fractions, the records of which contain a finite number of characters (digits).

Here are some examples of final decimals: 0.317 , 3.5 , 51.1020304958 , 230 032.45 .

However, not every common fraction can be represented as a finite decimal fraction. For example, the fraction 5/13 cannot be replaced by an equal fraction with one of the denominators 10, 100, ..., therefore, it cannot be converted to a final decimal fraction. We'll talk more about this in the theory section of converting ordinary fractions to decimal fractions.

Infinite decimals: periodic fractions and non-periodic fractions

In writing a decimal fraction after a decimal point, you can allow the possibility of an infinite number of digits. In this case, we will come to the consideration of the so-called infinite decimal fractions.

Definition.

Endless decimals- These are decimal fractions, in the record of which there is an infinite number of digits.

It is clear that we cannot write the infinite decimal fractions in full, therefore, in their recording they are limited to only a certain finite number of digits after the decimal point and put an ellipsis indicating an infinitely continuing sequence of digits. Here are some examples of infinite decimal fractions: 0.143940932…, 3.1415935432…, 153.02003004005…, 2.111111111…, 69.74152152152….

If you look closely at the last two endless decimal fractions, then in the fraction 2.111111111 ... the infinitely repeating number 1 is clearly visible, and in the fraction 69.74152152152 ..., starting from the third decimal place, the repeating group of numbers 1, 5 and 2 is clearly visible. Such infinite decimal fractions are called periodic.

Definition.

Periodic decimals(or simply periodic fractions) are infinite decimal fractions, in the record of which, starting from a certain decimal place, some digit or group of digits, which is called fraction period.

For example, the period of the periodic fraction 2.111111111… is the number 1, and the period of the fraction 69.74152152152… is a group of numbers like 152.

For infinite periodic decimal fractions, a special notation has been adopted. For brevity, we agreed to write the period once, enclosing it in parentheses. For example, the periodic fraction 2.111111111… is written as 2,(1) , and the periodic fraction 69.74152152152… is written as 69.74(152) .

It is worth noting that for the same periodic decimal fraction, you can specify different periods. For example, the periodic decimal 0.73333… can be considered as a fraction 0.7(3) with a period of 3, as well as a fraction 0.7(33) with a period of 33, and so on 0.7(333), 0.7 (3333), ... You can also look at the periodic fraction 0.73333 ... like this: 0.733(3), or like this 0.73(333), etc. Here, in order to avoid ambiguity and inconsistency, we agree to consider as the period of a decimal fraction the shortest of all possible sequences of repeating digits, and starting from the closest position to the decimal point. That is, the period of the decimal fraction 0.73333… will be considered a sequence of one digit 3, and the periodicity starts from the second position after the decimal point, that is, 0.73333…=0.7(3) . Another example: the periodic fraction 4.7412121212… has a period of 12, the periodicity starts from the third digit after the decimal point, that is, 4.7412121212…=4.74(12) .

Infinite decimal periodic fractions are obtained by converting to decimal fractions of ordinary fractions whose denominators contain prime factors other than 2 and 5.

Here it is worth mentioning periodic fractions with a period of 9. Here are examples of such fractions: 6.43(9) , 27,(9) . These fractions are another notation for periodic fractions with period 0, and it is customary to replace them with periodic fractions with period 0. To do this, period 9 is replaced by period 0, and the value of the next highest digit is increased by one. For example, a fraction with period 9 of the form 7.24(9) is replaced by a periodic fraction with period 0 of the form 7.25(0) or an equal final decimal fraction of 7.25. Another example: 4,(9)=5,(0)=5 . The equality of a fraction with a period of 9 and its corresponding fraction with a period of 0 is easily established after replacing these decimal fractions with their equal ordinary fractions.

Finally, let's take a closer look at infinite decimals, which do not have an infinitely repeating sequence of digits. They are called non-periodic.

Definition.

Non-recurring decimals(or simply non-periodic fractions) are infinite decimals with no period.

Sometimes non-periodic fractions have a form similar to that of periodic fractions, for example, 8.02002000200002 ... is a non-periodic fraction. In these cases, you should be especially careful to notice the difference.

Note that non-periodic fractions are not converted to ordinary fractions, infinite non-periodic decimal fractions represent irrational numbers.

Operations with decimals

One of the actions with decimals is comparison, and four basic arithmetic are also defined operations with decimals: addition, subtraction, multiplication and division. Consider separately each of the actions with decimal fractions.

Decimal Comparison essentially based on a comparison of ordinary fractions corresponding to the compared decimal fractions. However, converting decimal fractions to ordinary ones is a rather laborious operation, and infinite non-repeating fractions cannot be represented as an ordinary fraction, so it is convenient to use a bitwise comparison of decimal fractions. Bitwise comparison of decimals is similar to comparison of natural numbers. For more detailed information, we recommend that you study the article material comparison of decimal fractions, rules, examples, solutions.

Let's move on to the next step - multiplying decimals. Multiplication of final decimal fractions is carried out similarly to the subtraction of decimal fractions, rules, examples, solutions to multiplication by a column of natural numbers. In the case of periodic fractions, multiplication can be reduced to the multiplication of ordinary fractions. In turn, the multiplication of infinite non-periodic decimal fractions after their rounding is reduced to the multiplication of finite decimal fractions. We recommend further study of the material of the article multiplication of decimal fractions, rules, examples, solutions.

Decimals on the coordinate beam

There is a one-to-one correspondence between dots and decimals.

Let's figure out how points are constructed on the coordinate ray corresponding to a given decimal fraction.

We can replace finite decimal fractions and infinite periodic decimal fractions with ordinary fractions equal to them, and then construct the corresponding ordinary fractions on the coordinate ray. For example, a decimal fraction 1.4 corresponds to an ordinary fraction 14/10, therefore, the point with coordinate 1.4 is removed from the origin in the positive direction by 14 segments equal to a tenth of a single segment.

Decimal fractions can be marked on the coordinate beam, starting from the expansion of this decimal fraction into digits. For example, let's say we need to build a point with a coordinate of 16.3007 , since 16.3007=16+0.3+0.0007 , then we can get to this point by sequentially laying 16 unit segments from the origin of coordinates, 3 segments, the length of which equal to a tenth of a unit, and 7 segments, the length of which is equal to a ten thousandth of a unit segment.

This method of constructing decimal numbers on the coordinate beam allows you to get as close as you like to the point corresponding to an infinite decimal fraction.

It is sometimes possible to accurately plot a point corresponding to an infinite decimal. For example, , then this infinite decimal fraction 1.41421... corresponds to the point of the coordinate ray, remote from the origin by the length of the diagonal of a square with a side of 1 unit segment.

The reverse process of obtaining a decimal fraction corresponding to a given point on the coordinate beam is the so-called decimal measurement of a segment. Let's see how it is done.

Let our task be to get from the origin to a given point on the coordinate line (or infinitely approach it if it is impossible to get to it). With a decimal measurement of a segment, we can sequentially postpone any number of unit segments from the origin, then segments whose length is equal to a tenth of a single segment, then segments whose length is equal to a hundredth of a single segment, etc. By writing down the number of plotted segments of each length, we get the decimal fraction corresponding to a given point on the coordinate ray.

For example, to get to point M in the above figure, you need to set aside 1 unit segment and 4 segments, the length of which is equal to the tenth of the unit. Thus, the point M corresponds to the decimal fraction 1.4.

It is clear that the points of the coordinate beam, which cannot be reached during the decimal measurement, correspond to infinite decimal fractions.

Bibliography.

• Mathematics: studies. for 5 cells. general education institutions / N. Ya. Vilenkin, V. I. Zhokhov, A. S. Chesnokov, S. I. Shvartsburd. - 21st ed., erased. - M.: Mnemosyne, 2007. - 280 p.: ill. ISBN 5-346-00699-0.
• Mathematics. Grade 6: textbook. for general education institutions / [N. Ya. Vilenkin and others]. - 22nd ed., Rev. - M.: Mnemosyne, 2008. - 288 p.: ill. ISBN 978-5-346-00897-2.
• Algebra: textbook for 8 cells. general education institutions / [Yu. N. Makarychev, N. G. Mindyuk, K. I. Neshkov, S. B. Suvorova]; ed. S. A. Telyakovsky. - 16th ed. - M. : Education, 2008. - 271 p. : ill. - ISBN 978-5-09-019243-9.
• Gusev V. A., Mordkovich A. G. Mathematics (a manual for applicants to technical schools): Proc. allowance.- M.; Higher school, 1984.-351 p., ill.

Already in elementary school, students are faced with fractions. And then they appear in every topic. It is impossible to forget actions with these numbers. Therefore, you need to know all the information about ordinary and decimal fractions. These concepts are simple, the main thing is to understand everything in order.

Why are fractions needed?

The world around us consists of whole objects. Therefore, there is no need for shares. But everyday life constantly pushes people to work with parts of objects and things.

For example, chocolate consists of several slices. Consider the situation where its tile is formed by twelve rectangles. If you divide it into two, you get 6 parts. It will be well divided into three. But the five will not be able to give a whole number of slices of chocolate.

By the way, these slices are already fractions. And their further division leads to the appearance of more complex numbers.

What is a "fraction"?

This is a number consisting of parts of one. Outwardly, it looks like two numbers separated by a horizontal or slash. This feature is called fractional. The number written on the top (left) is called the numerator. The one on the bottom (right) is the denominator.

In fact, the fractional bar turns out to be a division sign. That is, the numerator can be called a dividend, and the denominator can be called a divisor.

What are the fractions?

In mathematics, there are only two types of them: ordinary and decimal fractions. Schoolchildren get acquainted with the first ones in the elementary grades, calling them simply “fractions”. The second learn in the 5th grade. That's when these names appear.

Common fractions are all those that are written as two numbers separated by a bar. For example, 4/7. Decimal is a number in which the fractional part has a positional notation and is separated from the integer with a comma. For example, 4.7. Students need to be clear that the two examples given are completely different numbers.

Every simple fraction can be written as a decimal. This statement is almost always true in reverse as well. There are rules that allow you to write a decimal fraction as an ordinary fraction.

What subspecies do these types of fractions have?

It is better to start in chronological order, as they are being studied. Common fractions come first. Among them, 5 subspecies can be distinguished.

Correct. Its numerator is always less than the denominator.

Wrong. Its numerator is greater than or equal to the denominator.

Reducible / irreducible. It can be either right or wrong. Another thing is important, whether the numerator and denominator have common factors. If there are, then they are supposed to divide both parts of the fraction, that is, to reduce it.

Mixed. An integer is assigned to its usual correct (incorrect) fractional part. And it always stands on the left.

Composite. It is formed from two fractions divided into each other. That is, it has three fractional features at once.

Decimals have only two subspecies:

final, that is, one in which the fractional part is limited (has an end);

infinite - a number whose digits after the decimal point do not end (they can be written endlessly).

How to convert decimal to ordinary?

If this is a finite number, then an association based on the rule is applied - as I hear, so I write. That is, you need to read it correctly and write it down, but without a comma, but with a fractional line.

As a hint about the required denominator, remember that it is always a one and a few zeros. The latter need to be written as many as the digits in the fractional part of the number in question.

How to convert decimal fractions to ordinary ones if their whole part is missing, that is, equal to zero? For example, 0.9 or 0.05. After applying the specified rule, it turns out that you need to write zero integers. But it is not indicated. It remains to write down only the fractional parts. For the first number, the denominator will be 10, for the second - 100. That is, the indicated examples will have numbers as answers: 9/10, 5/100. Moreover, the latter turns out to be possible to reduce by 5. Therefore, the result for it must be written 1/20.

How to make an ordinary fraction from a decimal if its integer part is different from zero? For example, 5.23 or 13.00108. Both examples read the integer part and write its value. In the first case, this is 5, in the second, 13. Then you need to move on to the fractional part. With them it is necessary to carry out the same operation. The first number has 23/100, the second has 108/100000. The second value needs to be reduced again. The answer is mixed fractions: 5 23/100 and 13 27/25000.

How to convert an infinite decimal to a common fraction?

If it is non-periodic, then such an operation cannot be carried out. This fact is due to the fact that each decimal fraction is always converted to either final or periodic.

The only thing that is allowed to be done with such a fraction is to round it. But then the decimal will be approximately equal to that infinite. It can already be turned into an ordinary one. But the reverse process: converting to decimal - will never give the initial value. That is, infinite non-periodic fractions are not translated into ordinary fractions. This must be remembered.

How to write an infinite periodic fraction in the form of an ordinary?

In these numbers, one or more digits always appear after the decimal point, which are repeated. They are called periods. For example, 0.3(3). Here "3" in the period. They are classified as rational, as they can be converted into ordinary fractions.

Those who have encountered periodic fractions know that they can be pure or mixed. In the first case, the period starts immediately from the comma. In the second, the fractional part begins with any numbers, and then the repetition begins.

The rule by which you need to write an infinite decimal in the form of an ordinary fraction will be different for these two types of numbers. It is quite easy to write pure periodic fractions as ordinary fractions. As with the final ones, they need to be converted: write the period into the numerator, and the number 9 will be the denominator, repeating as many times as there are digits in the period.

For example, 0,(5). The number does not have an integer part, so you need to immediately proceed to the fractional part. Write 5 in the numerator, and write 9 in the denominator. That is, the answer will be the fraction 5/9.

A rule on how to write a common decimal fraction that is a mixed fraction.

Look at the length of the period. So much 9 will have a denominator.

Write down the denominator: first nines, then zeros.

To determine the numerator, you need to write the difference of two numbers. All digits after the decimal point will be reduced, along with the period. Subtractable - it is without a period.

For example, 0.5(8) - write the periodic decimal fraction as a common fraction. The fractional part before the period is one digit. So zero will be one. There is also only one digit in the period - 8. That is, there is only one nine. That is, you need to write 90 in the denominator.

To determine the numerator from 58, you need to subtract 5. It turns out 53. For example, you will have to write 53/90 as an answer.

How are common fractions converted to decimals?

The simplest option is a number whose denominator is the number 10, 100, and so on. Then the denominator is simply discarded, and a comma is placed between the fractional and integer parts.

There are situations when the denominator easily turns into 10, 100, etc. For example, the numbers 5, 20, 25. It is enough to multiply them by 2, 5 and 4, respectively. Only it is necessary to multiply not only the denominator, but also the numerator by the same number.

For all other cases, a simple rule will come in handy: divide the numerator by the denominator. In this case, you may get two answers: a final or a periodic decimal fraction.

Operations with common fractions

Addition and subtraction

Students get to know them earlier than others. And at first the fractions have the same denominators, and then different. General rules can be reduced to such a plan.

Find the least common multiple of the denominators.

Write additional factors to all ordinary fractions.

Multiply the numerators and denominators by the factors defined for them.

Add (subtract) the numerators of fractions, and leave the common denominator unchanged.

If the numerator of the minuend is less than the subtrahend, then you need to find out whether we have a mixed number or a proper fraction.

In the first case, the integer part needs to take one. Add a denominator to the numerator of a fraction. And then do the subtraction.

In the second - it is necessary to apply the rule of subtraction from a smaller number to a larger one. That is, subtract the modulus of the minuend from the modulus of the subtrahend, and put the “-” sign in response.

Look carefully at the result of addition (subtraction). If you get an improper fraction, then it is supposed to select the whole part. That is, divide the numerator by the denominator.

Multiplication and division

For their implementation, fractions do not need to be reduced to a common denominator. This makes it easier to take action. But they still have to follow the rules.

When multiplying ordinary fractions, it is necessary to consider the numbers in the numerators and denominators. If any numerator and denominator have a common factor, then they can be reduced.

Multiply numerators.

Multiply the denominators.

If you get a reducible fraction, then it is supposed to be simplified again.

When dividing, you must first replace division with multiplication, and the divisor (second fraction) with a reciprocal (swap the numerator and denominator).

Then proceed as in multiplication (starting from step 1).

In tasks where you need to multiply (divide) by an integer, the latter is supposed to be written as an improper fraction. That is, with a denominator of 1. Then proceed as described above.



Operations with decimals

Addition and subtraction

Of course, you can always turn a decimal into a common fraction. And act according to the already described plan. But sometimes it is more convenient to act without this translation. Then the rules for their addition and subtraction will be exactly the same.

Equalize the number of digits in the fractional part of the number, that is, after the decimal point. Assign the missing number of zeros in it.

Write fractions so that the comma is under the comma.

Add (subtract) like natural numbers.

Remove the comma.

Multiplication and division

It is important that you do not need to append zeros here. Fractions are supposed to be left as they are given in the example. And then go according to plan.

For multiplication, you need to write fractions one under the other, not paying attention to commas.

Multiply like natural numbers.

Put a comma in the answer, counting from the right end of the answer as many digits as they are in the fractional parts of both factors.

To divide, you must first convert the divisor: make it a natural number. That is, multiply it by 10, 100, etc., depending on how many digits are in the fractional part of the divisor.

Multiply the dividend by the same number.

Divide a decimal by a natural number.

Put a comma in the answer at the moment when the division of the whole part ends.



What if there are both types of fractions in one example?

Yes, in mathematics there are often examples in which you need to perform operations on ordinary and decimal fractions. There are two possible solutions to these problems. You need to objectively weigh the numbers and choose the best one.

First way: represent ordinary decimals

It is suitable if, when dividing or converting, final fractions are obtained. If at least one number gives a periodic part, then this technique is prohibited. Therefore, even if you do not like working with ordinary fractions, you will have to count them.

The second way: write decimal fractions as ordinary

This technique is convenient if there are 1-2 digits in the part after the decimal point. If there are more of them, a very large ordinary fraction can turn out and decimal entries will allow you to calculate the task faster and easier. Therefore, it is always necessary to soberly evaluate the task and choose the simplest solution method.

Remember how in the very first lesson about decimal fractions, I said that there are numeric fractions that cannot be represented as decimals (see the lesson “ Decimal Fractions”)? We also learned how to factorize the denominators of fractions to check if there are any numbers other than 2 and 5.

So: I lied. And today we will learn how to translate absolutely any numerical fraction into a decimal. At the same time, we will get acquainted with a whole class of fractions with an infinite significant part.

A recurring decimal is any decimal that has:

1. The significant part consists of an infinite number of digits;
2. At certain intervals, the numbers in the significant part are repeated.

The set of repeated digits that make up the significant part is called the periodic part of the fraction, and the number of digits in this set is the period of the fraction. The remaining segment of the significant part, which does not repeat, is called the non-periodic part.

Since there are many definitions, it is worth considering in detail a few of these fractions:

This fraction occurs most often in problems. Non-periodic part: 0; periodic part: 3; period length: 1.

Non-periodic part: 0.58; periodic part: 3; period length: again 1.

Non-periodic part: 1; periodic part: 54; period length: 2.

Non-periodic part: 0; periodic part: 641025; period length: 6. For convenience, repeating parts are separated from each other by a space - in this solution it is not necessary to do so.

Non-periodic part: 3066; periodic part: 6; period length: 1.

As you can see, the definition of a periodic fraction is based on the concept significant part of a number. Therefore, if you forgot what it is, I recommend repeating it - see the lesson "".

Transition to periodic decimal

Consider an ordinary fraction of the form a / b . Let us decompose its denominator into simple factors. There are two options:

1. Only factors 2 and 5 are present in the expansion. These fractions are easily reduced to decimals - see the lesson " Decimal Fractions". We are not interested in such;
2. There is something else in the expansion besides 2 and 5. In this case, the fraction cannot be represented as a decimal, but it can be made into a periodic decimal.

To set a periodic decimal fraction, you need to find its periodic and non-periodic part. How? Convert the fraction to an improper one, and then divide the numerator by the denominator with a "corner".

In doing so, the following will happen:

1. Divide first whole part if it exists;
2. There may be several numbers after the decimal point;
3. After a while the numbers will start repeat.

That's all! Repeating digits after the decimal point are denoted by the periodic part, and what is in front - non-periodic.

Task. Convert ordinary fractions to periodic decimals:

All fractions without an integer part, so we simply divide the numerator by the denominator with a “corner”:

As you can see, the remnants are repeated. Let's write the fraction in the "correct" form: 1.733 ... = 1.7(3).

The result is a fraction: 0.5833 ... = 0.58(3).

We write in normal form: 4.0909 ... = 4, (09).

We get a fraction: 0.4141 ... = 0, (41).

Transition from periodic decimal to ordinary

Consider a periodic decimal X = abc (a 1 b 1 c 1). It is required to transfer it to the classic "two-story". To do this, follow four simple steps:

1. Find the period of the fraction, i.e. count how many digits are in the periodic part. Let it be number k;
2. Find the value of the expression X · 10 k . This is equivalent to shifting the decimal point a full period to the right - see the lesson " Multiplication and division of decimal fractions»;
3. Subtract the original expression from the resulting number. In this case, the periodic part is “burned out”, and remains common fraction;
4. Find X in the resulting equation. All decimal fractions are converted to ordinary.

Task. Convert to an ordinary improper fraction of a number:

• 9,(6);
• 32,(39);
• 0,30(5);
• 0,(2475).

Working with the first fraction: X = 9,(6) = 9.666 ...

The brackets contain only one digit, so the period k = 1. Next, we multiply this fraction by 10 k = 10 1 = 10. We have:

10X = 10 9.6666... ​​= 96.666...

Subtract the original fraction and solve the equation:

10X - X = 96.666 ... - 9.666 ... = 96 - 9 = 87;
9X=87;
X = 87/9 = 29/3.

Now let's deal with the second fraction. So X = 32,(39) = 32.393939 ...

Period k = 2, so we multiply everything by 10 k = 10 2 = 100:

100X = 100 32.393939 ... = 3239.3939 ...

Subtract the original fraction again and solve the equation:

100X - X = 3239.3939 ... - 32.3939 ... = 3239 - 32 = 3207;
99X = 3207;
X = 3207/99 = 1069/33.

Let's get to the third fraction: X = 0.30(5) = 0.30555 ... The scheme is the same, so I'll just give the calculations:

Period k = 1 ⇒ multiply everything by 10 k = 10 1 = 10;

10X = 10 0.30555... = 3.05555...
10X - X = 3.0555 ... - 0.305555 ... = 2.75 = 11/4;
9X = 11/4;
X = (11/4) : 9 = 11/36.

Finally, the last fraction: X = 0,(2475) = 0.2475 2475 ... Again, for convenience, the periodic parts are separated from each other by spaces. We have:

k = 4 ⇒ 10 k = 10 4 = 10,000;
10,000X = 10,000 0.2475 2475 = 2475.2475 ...
10,000X - X = 2475.2475 ... - 0.2475 2475 ... = 2475;
9999X = 2475;
X = 2475: 9999 = 25/101.

It is known that if the denominator P irreducible fraction in its canonical expansion has a prime factor not equal to 2 and 5, then this fraction cannot be represented as a finite decimal fraction. If in this case we try to write the original irreducible fraction as a decimal, dividing the numerator by the denominator, then the division process cannot end, because in the case of its completion after a finite number of steps, we would get a finite decimal fraction in the quotient, which contradicts the previously proved theorem. So in this case the decimal notation for a positive rational number is a= is represented as an infinite fraction.

For example, fraction = 0.3636... . It is easy to see that the remainders when dividing 4 by 11 are periodically repeated, therefore, the decimal places will be periodically repeated, i.e. it turns out infinite periodic decimal, which can be written as 0,(36).

Periodically repeating numbers 3 and 6 form a period. It may turn out that there are several digits between the comma and the beginning of the first period. These numbers form the pre-period. For example,

0.1931818... The process of dividing 17 by 88 is infinite. The numbers 1, 9, 3 form the pre-period; 1, 8 - period. The examples we have considered reflect a pattern, i.e. any positive rational number can be represented by either a finite or an infinite periodic decimal fraction.

Theorem 1. Let an ordinary fraction be irreducible and in the canonical expansion of the denominator n there is a prime factor different from 2 and 5. Then the ordinary fraction can be represented by an infinite periodic decimal fraction.

Proof. We already know that the process of dividing a natural number m to a natural number n will be endless. Let us show that it will be periodic. Indeed, when dividing m on the n residuals will be smaller n, those. numbers of the form 1, 2, ..., ( n- 1), which shows that the number of different residues is finite and therefore, starting from a certain step, some residue will be repeated, which will entail the repetition of the decimal places of the quotient, and the infinite decimal fraction becomes periodic.

There are two more theorems.

Theorem 2. If the expansion of the denominator of an irreducible fraction into prime factors does not include the numbers 2 and 5, then when this fraction is converted into an infinite decimal fraction, a pure periodic fraction will be obtained, i.e. A fraction whose period begins immediately after the decimal point.

Theorem 3. If the expansion of the denominator includes factors 2 (or 5) or both, then the infinite periodic fraction will be mixed, i.e. between the comma and the beginning of the period there will be several digits (pre-period), namely as many as the largest of the exponents of the factors 2 and 5.

Theorems 2 and 3 are invited to prove to the reader on their own.

28. Ways of passing from infinite periodic
decimal fractions to common fractions

Let there be a periodic fraction a= 0,(4), i.e. 0.4444... .

Let's multiply a by 10, we get

10a= 4.444…4…Þ 10 a = 4 + 0,444….

Those. ten a = 4 + a, we got the equation for a, solving it, we get: 9 a= 4 Þ a = .

Note that 4 is both the numerator of the resulting fraction and the period of the fraction 0,(4).

rule conversion to an ordinary fraction of a pure periodic fraction is formulated as follows: the numerator of the fraction is equal to the period, and the denominator consists of such a number of nines as there are digits in the period of the fraction.

Let us now prove this rule for a fraction whose period consists of P

a= . Let's multiply a on 10 n, we get:

10n × a = = + 0, ;

10n × a = + a;

(10n – 1) a = Þ a == .

So, the previously formulated rule is proved for any pure periodic fraction.

Let now given a fraction a= 0.605(43) - mixed periodic. Let's multiply a by 10 with such an indicator as how many digits are in the pre-period, i.e. by 10 3 , we get

10 3 × a= 605 + 0,(43) Þ 10 3 × a = 605 + = 605 + = = ,

those. 10 3 × a= .

rule conversion to an ordinary fraction of a mixed periodic fraction is formulated as follows: the numerator of the fraction is equal to the difference between the number written in digits before the beginning of the second period and the number written in digits before the beginning of the first period, the denominator consists of such a number of nines as there are digits in the period and such number of zeros how many digits are before the beginning of the first period.

Let us now prove this rule for a fraction whose preperiod consists of P digits, and a period of to digits. Let there be a periodic fraction

Denote in= ; r= ,

with= ; then with=in × 10k + r.

Let's multiply a by 10 with such an exponent how many digits are in the pre-period, i.e. on 10 n, we get:

a×10 n = + .

Taking into account the notation introduced above, we write:

a× 10n= in+ .

So, the rule formulated above is proved for any mixed periodic fraction.

Any infinite periodic decimal fraction is a form of writing some rational number.

For the sake of uniformity, sometimes a finite decimal is also considered an infinite periodic decimal with a period of "zero". For example, 0.27 = 0.27000...; 10.567 = 10.567000...; 3 = 3,000... .

Now the following statement becomes true: every rational number can be (and, moreover, in a unique way) expressed by an infinite decimal periodic fraction, and every infinite periodic decimal fraction expresses exactly one rational number (periodic decimal fractions with a period of 9 are not considered).