Root formulas. properties of square roots.

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In the previous lesson, we figured out what a square root is. It's time to figure out what are formulas for roots, what are root properties and what can be done about it all.

Root Formulas, Root Properties, and Rules for Actions with Roots- it's essentially the same thing. There are surprisingly few formulas for square roots. Which, of course, pleases! Rather, you can write a lot of all sorts of formulas, but only three are enough for practical and confident work with roots. Everything else flows from these three. Although many stray in the three formulas of the roots, yes ...

Let's start with the simplest. There she is:

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DEGREE WITH A RATIONAL INDICATOR,

POWER FUNCTION IV

§ 79. Extracting roots from a work and a quotient

Theorem 1. Root P th power of the product of positive numbers is equal to the product of the roots P -th degree of the factors, that is, when a > 0, b > 0 and natural P

n √ab = n √a n √b . (1)

Proof. Recall that the root P th power of a positive number ab there is a positive number P -th degree of which is equal to ab . Therefore, proving equality (1) is the same as proving the equality

(n √a n √b ) n = ab .

By the property of the degree of the product

(n √a n √b ) n = (n √a ) n (n √b ) n =.

But by definition of the root P th degree ( n √a ) n = a , (n √b ) n = b .

That's why ( n √a n √b ) n = ab . The theorem has been proven.

Requirement a > 0, b > 0 is essential only for even P , because for negative a and b and even P roots n √a and n √b not defined. If P odd, then formula (1) is valid for any a and b (both positive and negative).

Examples: √16 121 = √16 √121 = 4 11 = 44.

3 √-125 27 = 3 √-125 3 √27 = -5 3 = - 15

Formula (1) is useful when calculating the roots, when the root expression is represented as a product of exact squares. For example,

√153 2 -72 2 = √ (153+ 72) (153-72) = √225 81 = 15 9 = 135.

We proved Theorem 1 for the case when the radical sign on the left side of formula (1) is the product of two positive numbers. In fact, this theorem is true for any number of positive factors, that is, for any natural k > 2:

Consequence. Reading this identity from right to left, we get the following rule for multiplying roots with the same exponents;

To multiply roots with the same exponents, it is enough to multiply the root expressions, leaving the exponent of the root the same.

For example, √3 √8 √6 = √3 8 6 = √144 = 12.

Theorem 2. Root P th power of a fraction whose numerator and denominator are positive numbers is equal to the quotient of dividing the root of the same degree from the numerator by the root of the same degree from the denominator, that is, when a > 0 and b > 0

(2)

To prove equality (2) means to show that

According to the rule of raising a fraction to a power and determining the root n th degree we have:

Thus the theorem is proved.

Requirement a > 0 and b > 0 is essential only for even P . If P odd, then formula (2) is also true for negative values a and b .

Consequence. Reading identity from right to left, we get the following rule for dividing roots with the same exponents:

To divide roots with the same exponents, it is enough to divide the root expressions, leaving the exponent of the root the same.

For example,

Exercises

554. Where in the proof of Theorem 1 did we use the fact that a and b positive?

Why with an odd P formula (1) is also true for negative numbers a and b ?

At what values X the equality data is correct (No. 555-560):

555. √x 2 - 9 = √x -3 √x + 3 .

556. 4 √ (x - 2) (8 - x ) = 4 √x - 2 4 √ 8 - x

557. 3 √ (X + 1) (X - 5) = 3 √x +1 3 √x - 5 .

558. √ X (X + 1) (X + 2) = √ X √ (X + 1) √ (X + 2)

559. √ (x - a ) 3 = (√ x - a ) 3 .

560. 3 √ (X - 5) 2 = (3 √ X - 5 ) 2 .

561. Calculate:

a) √ 173 2 - 52 2 ; in) √ 200 2 - 56 2 ;

b) √ 3732 - 2522; G) √ 242,5 2 - 46,5 2 .

562. In a right triangle, the hypotenuse is 205 cm, and one of the legs is 84 cm. Find the other leg.

563. How many times:

555. X > 3. 556. 2 < X < 8. 557. X - any number. 558. X > 0. 559. X > a . 560. X - any number. 563. a) Three times.

Students always ask: “Why can't I use a calculator on a math exam? How to extract the square root of a number without a calculator? Let's try to answer this question.

How to extract the square root of a number without the help of a calculator?

Action square root extraction the opposite of squaring.

√81= 9 9 2 =81

If we take the square root of a positive number and square the result, we get the same number.

From small numbers that are exact squares of natural numbers, for example 1, 4, 9, 16, 25, ..., 100, square roots can be extracted verbally. Usually at school they teach a table of squares of natural numbers up to twenty. Knowing this table, it is easy to extract the square roots from the numbers 121,144, 169, 196, 225, 256, 289, 324, 361, 400. From numbers greater than 400, you can extract using the selection method using some tips. Let's try an example to consider this method.

Example: Extract the root of the number 676.

We notice that 20 2 \u003d 400, and 30 2 \u003d 900, which means 20< √676 < 900.

Exact squares of natural numbers end in 0; one; four; 5; 6; 9.
The number 6 is given by 4 2 and 6 2 .
So, if the root is taken from 676, then it is either 24 or 26.

It remains to check: 24 2 = 576, 26 2 = 676.

Answer: √676 = 26 .

More example: √6889 .

Since 80 2 \u003d 6400, and 90 2 \u003d 8100, then 80< √6889 < 90.
The number 9 is given by 3 2 and 7 2, then √6889 is either 83 or 87.

Check: 83 2 = 6889.

Answer: √6889 = 83 .

If you find it difficult to solve by the selection method, then you can factorize the root expression.

For example, find √893025.

Let's factorize the number 893025, remember, you did it in the sixth grade.

We get: √893025 = √3 6 ∙5 2 ∙7 2 = 3 3 ∙5 ∙7 = 945.

More example: √20736. Let's factorize the number 20736:

We get √20736 = √2 8 ∙3 4 = 2 4 ∙3 2 = 144.

Of course, factoring requires knowledge of divisibility criteria and factoring skills.

And finally, there is square root rule. Let's look at this rule with an example.

Calculate √279841.

To extract the root of a multi-digit integer, we split it from right to left into faces containing 2 digits each (there may be one digit in the left extreme face). Write like this 27'98'41

To get the first digit of the root (5), we extract the square root of the largest exact square contained in the first left face (27).
Then the square of the first digit of the root (25) is subtracted from the first face and the next face (98) is attributed (demolished) to the difference.
To the left of the received number 298, they write the double digit of the root (10), divide by it the number of all tens of the previously obtained number (29/2 ≈ 2), experience the quotient (102 ∙ 2 = 204 should be no more than 298) and write (2) after the first digit of the root.
Then the resulting quotient 204 is subtracted from 298, and the next facet (41) is attributed (demolished) to the difference (94).
To the left of the resulting number 9441, they write the double product of the digits of the root (52 ∙ 2 = 104), divide by this product the number of all tens of the number 9441 (944/104 ≈ 9), experience the quotient (1049 ∙ 9 = 9441) should be 9441 and write it down (9) after the second digit of the root.

We got the answer √279841 = 529.

Similarly extract roots of decimals. Only the radical number must be divided into faces so that the comma is between the faces.

Example. Find the value √0.00956484.

Just remember that if the decimal fraction has an odd number of decimal places, the square root is not exactly extracted from it.

So, now you have seen three ways to extract the root. Choose the one that suits you best and practice. To learn how to solve problems, you need to solve them. And if you have any questions, sign up for my lessons.

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Before the advent of calculators, students and teachers calculated square roots by hand. There are several ways to manually calculate the square root of a number. Some of them offer only an approximate solution, others give an exact answer.

Steps

Prime factorization

Factor the root number into factors that are square numbers. Depending on the root number, you will get an approximate or exact answer. Square numbers are numbers from which the whole square root can be taken. Factors are numbers that, when multiplied, give the original number. For example, the factors of the number 8 are 2 and 4, since 2 x 4 = 8, the numbers 25, 36, 49 are square numbers, since √25 = 5, √36 = 6, √49 = 7. Square factors are factors , which are square numbers. First, try to factorize the root number into square factors.

• For example, calculate the square root of 400 (manually). First try factoring 400 into square factors. 400 is a multiple of 100, that is, divisible by 25 - this is a square number. Dividing 400 by 25 gives you 16. The number 16 is also a square number. Thus, 400 can be factored into square factors of 25 and 16, that is, 25 x 16 = 400.
• This can be written as follows: √400 = √(25 x 16).

1. The square root of the product of some terms is equal to the product of the square roots of each term, that is, √(a x b) = √a x √b. Use this rule and take the square root of each square factor and multiply the results to find the answer.

   • In our example, take the square root of 25 and 16.
     • √(25 x 16)
     • √25 x √16
     • 5 x 4 = 20

2. If the radical number does not factor into two square factors (and it does in most cases), you will not be able to find the exact answer as an integer. But you can simplify the problem by decomposing the root number into a square factor and an ordinary factor (a number from which the whole square root cannot be taken). Then you will take the square root of the square factor and you will take the root of the ordinary factor.

   • For example, calculate the square root of the number 147. The number 147 cannot be factored into two square factors, but it can be factored into the following factors: 49 and 3. Solve the problem as follows:
     • = √(49 x 3)
     • = √49 x √3
     • = 7√3

3. If necessary, evaluate the value of the root. Now you can evaluate the value of the root (find an approximate value) by comparing it with the values ​​​​of the roots of square numbers that are closest (on both sides of the number line) to the root number. You will get the value of the root as a decimal fraction, which must be multiplied by the number behind the root sign.

   • Let's go back to our example. The root number is 3. The nearest square numbers to it are the numbers 1 (√1 = 1) and 4 (√4 = 2). Thus, the value of √3 lies between 1 and 2. Since the value of √3 is probably closer to 2 than to 1, our estimate is: √3 = 1.7. We multiply this value by the number at the root sign: 7 x 1.7 \u003d 11.9. If you do the calculations on a calculator, you get 12.13, which is pretty close to our answer.
     • This method also works with large numbers. For example, consider √35. The root number is 35. The nearest square numbers to it are the numbers 25 (√25 = 5) and 36 (√36 = 6). Thus, the value of √35 lies between 5 and 6. Since the value of √35 is much closer to 6 than it is to 5 (because 35 is only 1 less than 36), we can state that √35 is slightly less than 6. Checking with a calculator gives us the answer 5.92 - we were right.

4. Another way - factorize the root number into prime factors . Prime factors are numbers that are only divisible by 1 and themselves. Write the prime factors in a row and find pairs of identical factors. Such factors can be taken out of the sign of the root.

   • For example, calculate the square root of 45. We decompose the root number into prime factors: 45 \u003d 9 x 5, and 9 \u003d 3 x 3. Thus, √45 \u003d √ (3 x 3 x 5). 3 can be taken out of the root sign: √45 = 3√5. Now we can estimate √5.
   • Consider another example: √88.
     • = √(2 x 44)
     • = √ (2 x 4 x 11)
     • = √ (2 x 2 x 2 x 11). You got three multiplier 2s; take a couple of them and take them out of the sign of the root.
     • = 2√(2 x 11) = 2√2 x √11. Now we can evaluate √2 and √11 and find an approximate answer.

   Calculating the square root manually

   Using column division

   1. This method involves a process similar to long division and gives an accurate answer. First, draw a vertical line dividing the sheet into two halves, and then draw a horizontal line to the right and slightly below the top edge of the sheet to the vertical line. Now divide the root number into pairs of numbers, starting with the fractional part after the decimal point. So, the number 79520789182.47897 is written as "7 95 20 78 91 82, 47 89 70".

      • For example, let's calculate the square root of the number 780.14. Draw two lines (as shown in the picture) and write the number in the top left as "7 80, 14". It is normal that the first digit from the left is an unpaired digit. The answer (the root of the given number) will be written on the top right.

   2. Given the first pair of numbers (or one number) from the left, find the largest integer n whose square is less than or equal to the pair of numbers (or one number) in question. In other words, find the square number that is closest to, but less than, the first pair of numbers (or single number) from the left, and take the square root of that square number; you will get the number n. Write the found n at the top right, and write down the square n at the bottom right.

      • In our case, the first number on the left will be the number 7. Next, 4< 7, то есть 2 2 < 7 и n = 2. Напишите 2 сверху справа - это первая цифра в искомом квадратном корне. Напишите 2×2=4 справа снизу; вам понадобится это число для последующих вычислений.

   3. Subtract the square of the number n you just found from the first pair of numbers (or one number) from the left. Write the result of the calculation under the subtrahend (the square of the number n).

      • In our example, subtract 4 from 7 to get 3.

   4. Take down the second pair of numbers and write it down next to the value obtained in the previous step. Then double the number at the top right and write the result at the bottom right with "_×_=" appended.

      • In our example, the second pair of numbers is "80". Write "80" after the 3. Then, doubling the number from the top right gives 4. Write "4_×_=" from the bottom right.

   5. Fill in the blanks on the right.

      • In our case, if instead of dashes we put the number 8, then 48 x 8 \u003d 384, which is more than 380. Therefore, 8 is too large a number, but 7 is fine. Write 7 instead of dashes and get: 47 x 7 \u003d 329. Write 7 from the top right - this is the second digit in the desired square root of the number 780.14.

   6. Subtract the resulting number from the current number on the left. Write the result from the previous step below the current number on the left, find the difference and write it below the subtracted one.

      • In our example, subtract 329 from 380, which equals 51.

   7. Repeat step 4. If the demolished pair of numbers is the fractional part of the original number, then put the separator (comma) of the integer and fractional parts in the desired square root from the top right. On the left, carry down the next pair of numbers. Double the number at the top right and write the result at the bottom right with "_×_=" appended.

      • In our example, the next pair of numbers to be demolished will be the fractional part of the number 780.14, so put the separator of the integer and fractional parts in the desired square root from the top right. Demolish 14 and write down at the bottom left. Double the top right (27) is 54, so write "54_×_=" at the bottom right.

   8. Repeat steps 5 and 6. Find the largest number in place of dashes on the right (instead of dashes you need to substitute the same number) so that the multiplication result is less than or equal to the current number on the left.

      • In our example, 549 x 9 = 4941, which is less than the current number on the left (5114). Write 9 on the top right and subtract the result of the multiplication from the current number on the left: 5114 - 4941 = 173.

   9. If you need to find more decimal places for the square root, write a pair of zeros next to the current number on the left and repeat steps 4, 5 and 6. Repeat steps until you get the accuracy of the answer you need (number of decimal places).

   Understanding the process

   To master this method, imagine the number whose square root you need to find as the area of ​​​​the square S. In this case, you will look for the length of the side L of such a square. Calculate the value of L for which L² = S.

   Enter a letter for each digit in your answer. Denote by A the first digit in the value of L (the desired square root). B will be the second digit, C the third and so on.

   Specify a letter for each pair of leading digits. Denote by S a the first pair of digits in the value S, by S b the second pair of digits, and so on.

   Explain the connection of this method with long division. As in the division operation, where each time we are only interested in one next digit of the divisible number, when calculating the square root, we work with a pair of digits in sequence (to obtain the next one digit in the square root value).

   1. Consider the first pair of digits Sa of the number S (Sa = 7 in our example) and find its square root. In this case, the first digit A of the sought value of the square root will be such a digit, the square of which is less than or equal to S a (that is, we are looking for such an A that satisfies the inequality A² ≤ Sa< (A+1)²). В нашем примере, S1 = 7, и 2² ≤ 7 < 3²; таким образом A = 2.

      • Let's say we need to divide 88962 by 7; here the first step will be similar: we consider the first digit of the divisible number 88962 (8) and select the largest number that, when multiplied by 7, gives a value less than or equal to 8. That is, we are looking for a number d for which the inequality is true: 7 × d ≤ 8< 7×(d+1). В этом случае d будет равно 1.

I looked again at the plate ... And, let's go!

Let's start with a simple one:

Wait a minute. this, which means we can write it like this:

Got it? Here's the next one for you:

The roots of the resulting numbers are not exactly extracted? Don't worry, here are some examples:

But what if there are not two multipliers, but more? Same! The root multiplication formula works with any number of factors:

Now completely independent:

Answers: Well done! Agree, everything is very easy, the main thing is to know the multiplication table!

Root division

We figured out the multiplication of the roots, now let's proceed to the property of division.

Let me remind you that the formula in general looks like this:

And that means that the root of the quotient is equal to the quotient of the roots.

Well, let's look at examples:

That's all science. And here's an example:

Everything is not as smooth as in the first example, but as you can see, there is nothing complicated.

What if the expression looks like this:

You just need to apply the formula in reverse:

And here's an example:

You can also see this expression:

Everything is the same, only here you need to remember how to translate fractions (if you don’t remember, look at the topic and come back!). Remembered? Now we decide!

I am sure that you coped with everything, everything, now let's try to build roots in a degree.

Exponentiation

What happens if the square root is squared? It's simple, remember the meaning of the square root of a number - this is a number whose square root is equal to.

So, if we square a number whose square root is equal, then what do we get?

Well, of course, !

Let's look at examples:

Everything is simple, right? And if the root is in a different degree? It's OK!

Stick to the same logic and remember the properties and possible actions with powers.

Read the theory on the topic "" and everything will become extremely clear to you.

For example, here's an expression:

In this example, the degree is even, but what if it is odd? Again, apply the power properties and factor everything:

With this, everything seems to be clear, but how to extract the root from a number in a degree? Here, for example, is this:

Pretty simple, right? What if the degree is greater than two? We follow the same logic using the properties of degrees:

Well, is everything clear? Then solve your own examples:

And here are the answers:

Introduction under the sign of the root

What we just have not learned to do with the roots! It remains only to practice entering the number under the root sign!

It's quite easy!

Let's say we have a number

What can we do with it? Well, of course, hide the triple under the root, while remembering that the triple is the square root of!

Why do we need it? Yes, just to expand our capabilities when solving examples:

How do you like this property of roots? Makes life much easier? For me, that's right! Only we must remember that we can only enter positive numbers under the square root sign.

Try this example for yourself:
Did you manage? Let's see what you should get:

Well done! You managed to enter a number under the root sign! Let's move on to something equally important - consider how to compare numbers containing a square root!

Root Comparison

Why should we learn to compare numbers containing a square root?

Very simple. Often, in large and long expressions encountered in the exam, we get an irrational answer (do you remember what it is? We already talked about this today!)

We need to place the received answers on the coordinate line, for example, to determine which interval is suitable for solving the equation. And this is where the snag arises: there is no calculator on the exam, and without it, how to imagine which number is larger and which is smaller? That's it!

For example, determine which is greater: or?

You won't say right off the bat. Well, let's use the parsed property of adding a number under the root sign?

Then forward:

Well, obviously, the larger the number under the sign of the root, the larger the root itself!

Those. if means .

From this we firmly conclude that And no one will convince us otherwise!

Extracting roots from large numbers

Before that, we introduced a factor under the sign of the root, but how to take it out? You just need to factor it out and extract what is extracted!

It was possible to go the other way and decompose into other factors:

Not bad, right? Any of these approaches is correct, decide how you feel comfortable.

Factoring is very useful when solving such non-standard tasks as this one:

We don't get scared, we act! We decompose each factor under the root into separate factors:

And now try it yourself (without a calculator! It will not be on the exam):

Is this the end? We don't stop halfway!

That's all, it's not all that scary, right?

Happened? Well done, you're right!

Now try this example:

And an example is a tough nut to crack, so you can’t immediately figure out how to approach it. But we, of course, are in the teeth.

Well, let's start factoring, shall we? Immediately, we note that you can divide a number by (recall the signs of divisibility):

And now, try it yourself (again, without a calculator!):

Well, did it work? Well done, you're right!

Summing up

1. The square root (arithmetic square root) of a non-negative number is a non-negative number whose square is equal.
   .
2. If we just take the square root of something, we always get one non-negative result.
3. Arithmetic root properties:
4. When comparing square roots, it must be remembered that the larger the number under the sign of the root, the larger the root itself.

How do you like the square root? All clear?

We tried to explain to you without water everything you need to know in the exam about the square root.

It's your turn. Write to us whether this topic is difficult for you or not.

Did you learn something new or everything was already so clear.

Write in the comments and good luck on the exams!