GRE Math : How to find an exponent from a rational number

Study concepts, example questions & explanations for GRE Math

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Example Questions

Example Question #1 : How To Find An Exponent From A Rational Number

 

find x

8x=2x+6

Possible Answers:

-1

2 or -1

2

3

4

Correct answer:

3

Explanation:

8 = 23

(23)x = 23x

 

23x = 2x+6  <- when the bases are the same, you can set the exponents equal to each other and solve for x

3x=x+6

2x=6

x=3

Example Question #2 : How To Find An Exponent From A Rational Number

Compare 3^{6}\(\displaystyle 3^{6}\) and 27^{2}\(\displaystyle 27^{2}\).

Possible Answers:

3^{6} < 27^{2}\(\displaystyle 3^{6} < 27^{2}\)

The relationship cannot be determined from the information given.

3^{6} > 27^{2}\(\displaystyle 3^{6} > 27^{2}\)

3^{6} = 27^{2}\(\displaystyle 3^{6} = 27^{2}\)

Correct answer:

3^{6} = 27^{2}\(\displaystyle 3^{6} = 27^{2}\)

Explanation:

First rewrite the two expressions so that they have the same base, and then compare their exponents.

27 = 3^{3}\(\displaystyle 27 = 3^{3}\)   

27^2 = (3^{3})^2\(\displaystyle 27^2 = (3^{3})^2\)

Combine exponents by multiplying: (3^{3})^2 = 3^6\(\displaystyle (3^{3})^2 = 3^6\)

This is the same as the first given expression, so the two expressions are equal.

Example Question #23 : Gre Quantitative Reasoning

Solve for \(\displaystyle x\)

\(\displaystyle 2^x=32\)

Possible Answers:

\(\displaystyle 5\)

\(\displaystyle 4\)

\(\displaystyle 6\)

\(\displaystyle 8\)

\(\displaystyle 7\)

Correct answer:

\(\displaystyle 5\)

Explanation:

\(\displaystyle 32\) can be written as \(\displaystyle 2^5.\) 

Since there is a common base of \(\displaystyle 2\), we can say

 \(\displaystyle 2^x=2^5\) or \(\displaystyle x=5\)

Example Question #23 : Exponents

Solve for \(\displaystyle x\).

\(\displaystyle 3^x=\frac{1}{9}\)

 

Possible Answers:

\(\displaystyle \frac{1}{2}\)

\(\displaystyle 3\)

\(\displaystyle -2\)

\(\displaystyle -\frac{1}{2}\)

\(\displaystyle 2\)

Correct answer:

\(\displaystyle -2\)

Explanation:

The basees don't match.

However: 

\(\displaystyle \frac{1}{3}=3^{-1}\) thus we can rewrite the expression as \(\displaystyle \frac{1}{9}=\left(\frac{1}{3}\right)^2\).

Anything raised to negative power means \(\displaystyle 1\) over the base raised to the postive exponent. 

So, \(\displaystyle (3^{-1})^{2}\)\(\displaystyle =3^{-2}\)\(\displaystyle x=-2\)

Example Question #3 : Exponents And Rational Numbers

Solve for \(\displaystyle x\).

\(\displaystyle \frac{1}{4}^x=256\)

Possible Answers:

\(\displaystyle -\frac{1}{4}\)

\(\displaystyle -4\)

\(\displaystyle 4\)

\(\displaystyle 256\)

\(\displaystyle \frac{1}{4}\)

Correct answer:

\(\displaystyle -4\)

Explanation:

The bases don't match.

However: 

\(\displaystyle 4=\frac{1}{4}^{-1}\) and we recognize that \(\displaystyle 256=4^4\).

Anything raised to negative power means \(\displaystyle 1\) over the base raised to the postive exponent. 

\(\displaystyle \left(\frac{1}{4}^{-1}\right)^4=\frac{1}{4}^{-4}\).  

 \(\displaystyle x=-4\)

Example Question #4 : Exponents And Rational Numbers

Solve for \(\displaystyle x.\)

\(\displaystyle 2^{x+1}=128\)

Possible Answers:

\(\displaystyle 8\)

\(\displaystyle 9\)

\(\displaystyle 7\)

\(\displaystyle 6\)

\(\displaystyle 5\)

Correct answer:

\(\displaystyle 6\)

Explanation:

Recall that \(\displaystyle 128=2^7\)

With same base, we can write this equation: 

\(\displaystyle x+1=7\)

By subtracting \(\displaystyle 1\) on both sides, \(\displaystyle x=6\)

 

Example Question #32 : Algebra

Solve for \(\displaystyle x\).

\(\displaystyle 2^{x^2+4}=32\)

Possible Answers:

\(\displaystyle 5\)

\(\displaystyle -1, 1\)

\(\displaystyle -5\)

\(\displaystyle 1\)

\(\displaystyle -1\)

Correct answer:

\(\displaystyle -1, 1\)

Explanation:

Since \(\displaystyle 32=2^5\) we can rewrite the expression.

With same base, let's set up an equation of \(\displaystyle x^2+4=5\).

By subtracting \(\displaystyle 4\) on both sides, we get \(\displaystyle x^2=1\).

Take the square root of both sides we get BOTH \(\displaystyle 1\) and \(\displaystyle -1\)

Example Question #32 : Gre Quantitative Reasoning

Solve for \(\displaystyle x\).

\(\displaystyle 5^x=25^4\)

Possible Answers:

\(\displaystyle 6\)

\(\displaystyle 4\)

\(\displaystyle 10\)

\(\displaystyle 8\)

\(\displaystyle 5\)

Correct answer:

\(\displaystyle 8\)

Explanation:

They don't have the same base, however: \(\displaystyle 25=5^2\).

Then \(\displaystyle 25^4=(5^2)^4\). You would multiply the \(\displaystyle 2\) and the \(\displaystyle 4\) instead of adding.

\(\displaystyle 2\cdot 4=8\)

\(\displaystyle x=8\)

Example Question #33 : Gre Quantitative Reasoning

Solve for \(\displaystyle x\).

\(\displaystyle 4^{2x}=16^6\)

Possible Answers:

\(\displaystyle 12\)

\(\displaystyle 4\)

\(\displaystyle 8\)

\(\displaystyle 6\)

\(\displaystyle 10\)

Correct answer:

\(\displaystyle 6\)

Explanation:

There are two ways to go about this.

Method \(\displaystyle 1:\)

They don't have the same bases however: \(\displaystyle 16=4^2\). Then \(\displaystyle 16^6=(4^2)^6\)

You would multiply the \(\displaystyle 2\) and the \(\displaystyle 6\) instead of adding. We have \(\displaystyle 2\cdot 6=2x\)

Divide \(\displaystyle 2\) on both sides to get \(\displaystyle x=6\).

 

Method \(\displaystyle 2\):

We can change the base from \(\displaystyle 4\) to \(\displaystyle 16.\)

\(\displaystyle 4^{2x}=(4^2)^x=16^x\) 

This is the basic property of the product of power exponents. 

We have the same base so basically \(\displaystyle x=6\)

Example Question #3 : Exponents And Rational Numbers

Solve for \(\displaystyle x\).

\(\displaystyle 1024^x=2\)

Possible Answers:

\(\displaystyle -\frac{1}{10}\)

\(\displaystyle -10\)

\(\displaystyle 2\)

\(\displaystyle \frac{1}{10}\)

\(\displaystyle 10\)

Correct answer:

\(\displaystyle \frac{1}{10}\)

Explanation:

Since we can write \(\displaystyle 1024^x=(2^{10})^x\)

With same base we can set up an equation of \(\displaystyle 10x=1\) 

Divide both sides by \(\displaystyle 10\) and we get \(\displaystyle x=\frac{1}{10}\)

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