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

Study concepts, example questions & explanations for PSAT Math

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Example Questions

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

Com_exp_1

Which of the following lists the above quantities from least to greatest?

Possible Answers:

I, IV, III, II

II, III, I, IV

I, III, II, IV

I, IV, II, III

IV, III, II, I

Correct answer:

I, III, II, IV

Explanation:

Com_exp_2

Com_exp_3

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

Solve for \(\displaystyle x\).

2^{x}= 64\(\displaystyle 2^{x}= 64\)

Possible Answers:

\(\displaystyle x=64\)

\(\displaystyle x=64^{2}\)

\(\displaystyle x=6\)

\(\displaystyle x=-6\)

\(\displaystyle x=5\)

Correct answer:

\(\displaystyle x=6\)

Explanation:

Since 2^{x}= 2^{6}\(\displaystyle 2^{x}= 2^{6}\)

Hence \(\displaystyle x=6\)

Example Question #3 : How To Find An Exponent From A Rational Number

Simplify:

 

\(\displaystyle \sqrt[3]{x^{10}y^{8}z^{9}}\)

Possible Answers:

\(\displaystyle \sqrt[3]{xy^{2}}\)

\(\displaystyle x^{3}y^{2}z^{3}\sqrt[3]{xy^{2}}\)

\(\displaystyle x^{3}y^{2}z^{3}^{\sqrt[3]{xyz}}\)

\(\displaystyle x^{3}y^{3}z^{3}\)

\(\displaystyle x^{3}y^{2}z^{3}\)

Correct answer:

\(\displaystyle x^{3}y^{2}z^{3}\sqrt[3]{xy^{2}}\)

Explanation:

\(\displaystyle \sqrt[3]{x^{9}xy^{6}y^{2}z^{9}}=x^{3}y^{2}z^{3}\sqrt[3]{xy^{2}}\)

Example Question #4 : How To Find An Exponent From A Rational Number

Solve for \(\displaystyle x\):

\(\displaystyle \sqrt{x} -1= 4\)

Possible Answers:

\(\displaystyle 36\)

\(\displaystyle 25\)

\(\displaystyle 5\)

\(\displaystyle 9\)

\(\displaystyle 5\)

Correct answer:

\(\displaystyle 25\)

Explanation:

From the equation one can see that

\(\displaystyle \sqrt{x} = 5\)

Hence \(\displaystyle x\) must be equal to 25.

Example Question #5 : How To Find An Exponent From A Rational Number

Evaluate:

\(\displaystyle \log 0.1=\)

Possible Answers:

\(\displaystyle -1\)

\(\displaystyle -2\)

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

\(\displaystyle 1\)

\(\displaystyle 10\)

Correct answer:

\(\displaystyle -1\)

Explanation:

\(\displaystyle 0.1= 10^{-1}\)

\(\displaystyle \log 0.1=-1\)

Example Question #6 : How To Find An Exponent From A Rational Number

Solve for \(\displaystyle x\).

 

\(\displaystyle \log_{2}x=4\)

Possible Answers:

\(\displaystyle 16\)

\(\displaystyle 32\)

\(\displaystyle 64\)

\(\displaystyle 8\)

\(\displaystyle 1\)

Correct answer:

\(\displaystyle 16\)

Explanation:

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

Example Question #7 : How To Find An Exponent From A Rational Number

\(\displaystyle \log 3=a\)

and

\(\displaystyle \log 5=b\)

 

Find \(\displaystyle \log 15\)

Possible Answers:

\(\displaystyle a^{2}\)

\(\displaystyle a\)

\(\displaystyle b^{2}\)

\(\displaystyle b\)

\(\displaystyle a+b\)

Correct answer:

\(\displaystyle a+b\)

Explanation:

\(\displaystyle \log 15=\log (5\cdot 3)=\log 5+\log 3\)

Hence the correct answer is \(\displaystyle a+b\).

Example Question #8 : How To Find An Exponent From A Rational Number

Solve for \(\displaystyle x\).

\(\displaystyle \log (x+3)+\log x=1\)

Possible Answers:

\(\displaystyle -5\ or\ 2\)

\(\displaystyle 10\)

\(\displaystyle 5\)

\(\displaystyle 5\ or\ -2\)

\(\displaystyle 2\)

Correct answer:

\(\displaystyle 2\)

Explanation:

Use the rules of logarithms to combine terms.

\(\displaystyle \log (x(x+3))=1\)

Hence, \(\displaystyle x(x+3)=10^{1}\)

\(\displaystyle x^{2}+3x=10\)

\(\displaystyle x^2+3x-10=0\)

By fatoring we get \(\displaystyle (x+5)(x-2)=0\)

Hence \(\displaystyle x=-5\ or\ 2\).

However, you cannot take the logarithm of a negative number. Thus, the only value for \(\displaystyle x\) is \(\displaystyle 2\).

Example Question #9 : How To Find An Exponent From A Rational Number

If a piece of pie is cut into 3 sections, and each of those pieces is further cut into three sections, then those pieces are cut into three sections, how many (tiny) pieces of pie are there?

Possible Answers:

12

27

36

40

125

Correct answer:

27

Explanation:

The answer is 33 = 27

Example Question #10 : How To Find An Exponent From A Rational Number

Solve for \(\displaystyle x\).

\(\displaystyle \log _{9}x=\frac{1}{2}\)

Possible Answers:

\(\displaystyle 9\)

\(\displaystyle 81\)

\(\displaystyle 3\)

\(\displaystyle 1\)

\(\displaystyle 27\)

Correct answer:

\(\displaystyle 3\)

Explanation:

\(\displaystyle \log _{9}x=\frac{1}{2}\)

\(\displaystyle x = 9^{\frac{1}{2}}= 3\)

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