PSAT Math : Exponential Ratios and Rational Numbers

Study concepts, example questions & explanations for PSAT Math

varsity tutors app store varsity tutors android store

Example Questions

Example Question #11 : Exponential Ratios And Rational Numbers

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:

36

40

125

12

27

Correct answer:

27

Explanation:

The answer is 33 = 27

Example Question #12 : Exponential Ratios And Rational Numbers

Solve for \(\displaystyle x\).

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

Possible Answers:

\(\displaystyle 1\)

\(\displaystyle 3\)

\(\displaystyle 27\)

\(\displaystyle 9\)

\(\displaystyle 81\)

Correct answer:

\(\displaystyle 3\)

Explanation:

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

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

Example Question #11 : Exponential Ratios And Rational Numbers

\(\displaystyle Simplify: \frac{\sqrt[3]{81}}{\sqrt[3]{3}}\)

Possible Answers:

\(\displaystyle \sqrt[3]{\frac{81}{3}}= \sqrt[3]{27}= 3\)

\(\displaystyle \frac{\sqrt[3]{81}}{\sqrt[3]{3}} = \frac{3}{1}= 3\)

\(\displaystyle \frac{\sqrt[3]{9\star 9}}{\sqrt[3]{9}}= \sqrt[3]{9}\)

\(\displaystyle \frac{81}{3}= 27\)

\(\displaystyle \sqrt[3]{81\star 3}= \sqrt[3]{243} = 6.24\)

Correct answer:

\(\displaystyle \sqrt[3]{\frac{81}{3}}= \sqrt[3]{27}= 3\)

Explanation:

\(\displaystyle \frac{\sqrt[3]{81}}{\sqrt[3]{3}} = \sqrt[3]{\frac{81}{3}}= \sqrt[3]{27}= 3\)

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

 

 

 

 

Rationalize the denominator:

\(\displaystyle \frac{1}{2-\sqrt{3}}\)

Possible Answers:

\(\displaystyle 2+\sqrt{3}\)

\(\displaystyle \sqrt{3}\)

\(\displaystyle \frac{2+\sqrt{3}}{7}\)

\(\displaystyle 2\)

\(\displaystyle 2-\sqrt{3}\)

Correct answer:

\(\displaystyle 2+\sqrt{3}\)

Explanation:

The conjugate of \(\displaystyle 2-\sqrt{3}\) is  \(\displaystyle 2+\sqrt{3}\).

Now multiply both the numerator and the denominator by \(\displaystyle 2+\sqrt{3}\)

and you get:

 \(\displaystyle \frac{2+\sqrt{3}}\left ({2-\sqrt{3}} \right )\left ( 2+\sqrt{3} \right )\)

\(\displaystyle \left ( 2-\sqrt{3} \right )\left ( 2+\sqrt{3} \right ) =2^{2}-\left ( \sqrt{3} \right )^{2} =4-3 = 1\)

Hence we get

\(\displaystyle \frac{2+\sqrt{3}}{1} = 2+\sqrt{3}\)

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

Solve for \(\displaystyle x\):

\(\displaystyle 3^{x-5} = \frac{1}{27}\)

Possible Answers:

\(\displaystyle 0\)

\(\displaystyle 3\)

\(\displaystyle 27\)

\(\displaystyle 9\)

\(\displaystyle 2\)

Correct answer:

\(\displaystyle 2\)

Explanation:

\(\displaystyle 3^{x-5}=\frac{1}{27}=3^{-3}\)

\(\displaystyle x-5=-3\) 

\(\displaystyle x=2\)

Example Question #12 : Exponential Ratios And Rational Numbers

Solve for \(\displaystyle x\).

\(\displaystyle \log _{5}x=2\)

Possible Answers:

\(\displaystyle 2\)

\(\displaystyle 25\)

\(\displaystyle 125\)

\(\displaystyle 5\)

\(\displaystyle 10\)

Correct answer:

\(\displaystyle 25\)

Explanation:

\(\displaystyle log_5{x} = 2\)

\(\displaystyle x = 5^{2} = 25\)

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

If,

 \(\displaystyle log_a{120}= log_b{120}\)

What does \(\displaystyle a=?\)

Possible Answers:

\(\displaystyle b=10\)

\(\displaystyle ab=10\)

\(\displaystyle a=b\)

\(\displaystyle \frac{a}{b}= 120\)

\(\displaystyle a=10\)

Correct answer:

\(\displaystyle a=b\)

Explanation:

If  \(\displaystyle log_a{x} = log_b{x}\),

 then \(\displaystyle a=b\).

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

\(\displaystyle \sqrt{x+3} -x = 1\)

Possible Answers:

\(\displaystyle x=0\)

\(\displaystyle x=9\)

\(\displaystyle x=4\)

\(\displaystyle x=13\)

\(\displaystyle x=1\)

Correct answer:

\(\displaystyle x=1\)

Explanation:

From the equation in the problem statement

\(\displaystyle \sqrt{x+3} = x + 1\)

Now squaring both sides we get

\(\displaystyle x+3=x^{2}+2x+1\)  this is a quadratic equation which equals

\(\displaystyle x^{2}+x -2 = 0\)

and the factors of this equation are

\(\displaystyle \left ( x+2 \right )\left ( x-1 \right )\)

This gives us \(\displaystyle x=-2\ or\ x=1\).

However, if we plug these solutions back into the original equation, \(\displaystyle -2\)  does not create an equality. Therefore, \(\displaystyle x=-2\) is an extraneous solution.

 

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

For some positive integer \(\displaystyle x\), if \(\displaystyle x^3=27\), what is the value of \(\displaystyle (x+2)^3\)?

Possible Answers:

\(\displaystyle 64\)

\(\displaystyle 125\)

\(\displaystyle -25\)

\(\displaystyle 25\)

\(\displaystyle 8\)

Correct answer:

\(\displaystyle 125\)

Explanation:

If \(\displaystyle x^3=27\), then \(\displaystyle x\) must equal 3 (Note that \(\displaystyle x\) cannot be -3 because you need it to be positive. 

Now, plug \(\displaystyle x=3\) into the new equation \(\displaystyle (x+2)^3\):

\(\displaystyle =(3+2)^3\)

\(\displaystyle =5^3\)

\(\displaystyle =125\)

 

Learning Tools by Varsity Tutors