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Understanding n Th roots

Suppose we know that a 3 = 8. We want to find what number raised to the 3rd power is equal to 8. Since 2 3 = 8 , we say that 2 is the cube root of 8.

The n th root of a is a number that, when raised to the n th power, gives a . For example, −3 is the 5th root of −243 because ( −3 ) 5 = −243. If a is a real number with at least one n th root, then the principal n th root    of a is the number with the same sign as a that, when raised to the n th power, equals a .

The principal n th root of a is written as a n , where n is a positive integer greater than or equal to 2. In the radical expression, n is called the index    of the radical.

Principal n Th root

If a is a real number with at least one n th root, then the principal n th root    of a , written as a n , is the number with the same sign as a that, when raised to the n th power, equals a . The index    of the radical is n .

Simplifying n Th roots

Simplify each of the following:

  1. −32 5
  2. 4 4 1 , 024 4
  3. 8 x 6 125 3
  4. 8 3 4 48 4
  1. −32 5 = −2 because ( −2 ) 5 = −32
  2. First, express the product as a single radical expression. 4,096 4 = 8 because 8 4 = 4,096
  3. 8 x 6 3 125 3 Write as quotient of two radical expressions . 2 x 2 5 Simplify .
  4. 8 3 4 2 3 4 Simplify to get equal radicands . 6 3 4   Add .
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  1. −216 3
  2. 3 80 4 5 4
  3. 6 9 , 000 3 + 7 576 3
  1. −6
  2. 6
  3. 88 9 3
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Using rational exponents

Radical expressions can also be written without using the radical symbol. We can use rational (fractional) exponents. The index must be a positive integer. If the index n is even, then a cannot be negative.

a 1 n = a n

We can also have rational exponents with numerators other than 1. In these cases, the exponent must be a fraction in lowest terms. We raise the base to a power and take an n th root. The numerator tells us the power and the denominator tells us the root.

a m n = ( a n ) m = a m n

All of the properties of exponents that we learned for integer exponents also hold for rational exponents.

Rational exponents

Rational exponents are another way to express principal n th roots. The general form for converting between a radical expression with a radical symbol and one with a rational exponent is

a m n = ( a n ) m = a m n

Given an expression with a rational exponent, write the expression as a radical.

  1. Determine the power by looking at the numerator of the exponent.
  2. Determine the root by looking at the denominator of the exponent.
  3. Using the base as the radicand, raise the radicand to the power and use the root as the index.

Writing rational exponents as radicals

Write 343 2 3 as a radical. Simplify.

The 2 tells us the power and the 3 tells us the root.

343 2 3 = ( 343 3 ) 2 = 343 2 3

We know that 343 3 = 7 because 7 3 = 343. Because the cube root is easy to find, it is easiest to find the cube root before squaring for this problem. In general, it is easier to find the root first and then raise it to a power.

343 2 3 = ( 343 3 ) 2 = 7 2 = 49

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Write 9 5 2 as a radical. Simplify.

( 9 ) 5 = 3 5 = 243

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Writing radicals as rational exponents

Write 4 a 2 7 using a rational exponent.

The power is 2 and the root is 7, so the rational exponent will be 2 7 . We get 4 a 2 7 . Using properties of exponents, we get 4 a 2 7 = 4 a −2 7 .

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Write x ( 5 y ) 9 using a rational exponent.

x ( 5 y ) 9 2

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Questions & Answers

An investment account was opened with an initial deposit of $9,600 and earns 7.4% interest, compounded continuously. How much will the account be worth after 15 years?
Kala Reply
lim x to infinity e^1-e^-1/log(1+x)
given eccentricity and a point find the equiation
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12, 17, 22.... 25th term
Alexandra Reply
12, 17, 22.... 25th term
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I'm 13 and I understand it great
I am 1 year old but I can do it! 1+1=2 proof very hard for me though.
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find the 15th term of the geometric sequince whose first is 18 and last term of 387
Jerwin Reply
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The given of f(x=x-2. then what is the value of this f(3) 5f(x+1)
virgelyn Reply
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how do they get the third part x = (32)5/4
kinnecy Reply
make 5/4 into a mixed number, make that a decimal, and then multiply 32 by the decimal 5/4 turns out to be
can someone help me with some logarithmic and exponential equations.
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it think it's written 20/(X-6)^2 so it's 20 divided by X-6 squared
I'm not sure why it wrote it the other way
I got X =-6
ok. so take the square root of both sides, now you have plus or minus the square root of 20= x-6
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Commplementary angles
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Abdirahman Reply
algebra 2 Inequalities:If equation 2 = 0 it is an open set?
Kim Reply
or infinite solutions?
The answer is neither. The function, 2 = 0 cannot exist. Hence, the function is undefined.
Embra Reply
Practice Key Terms 6

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Source:  OpenStax, College algebra. OpenStax CNX. Feb 06, 2015 Download for free at https://legacy.cnx.org/content/col11759/1.3
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