ISEE Upper Level Quantitative : How to find the radius of a sphere

Study concepts, example questions & explanations for ISEE Upper Level Quantitative

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Example Questions

Example Question #1 : How To Find The Radius Of A Sphere

The volume of a sphere is one cubic yard. Give its radius in inches.

Possible Answers:

\(\displaystyle \frac{ \sqrt{3 \pi } } {2 \pi} \textrm{ in}\)

\(\displaystyle \frac{ 18\sqrt[3]{6 \pi^{2}} } {\pi} \textrm{ in}\)

\(\displaystyle \frac{ 6\sqrt[3]{6 \pi^{2}} } {\pi} \textrm{ in}\)

\(\displaystyle \frac{ 18\sqrt{3 \pi } } {\pi} \textrm{ in}\)

\(\displaystyle \frac{ \sqrt[3]{6 \pi^{2}} } {2\pi}\textrm{ in}\)

Correct answer:

\(\displaystyle \frac{ 18\sqrt[3]{6 \pi^{2}} } {\pi} \textrm{ in}\)

Explanation:

The volume \(\displaystyle V\) of a sphere with radius \(\displaystyle r\) is 

\(\displaystyle V = \frac{4}{3} \pi r^{3}\) .

To find the radius in yards, we set \(\displaystyle V = 1\) and solve for \(\displaystyle r\).

\(\displaystyle 1 = \frac{4}{3} \pi r^{3}\)

\(\displaystyle \frac{3}{4} \cdot 1 =\frac{3}{4} \cdot \frac{4}{3} \pi r^{3}\)

\(\displaystyle \frac{3}{4} = \pi r^{3}\)

\(\displaystyle \frac{1}{\pi} \cdot \frac{3}{4} =\frac{1}{\pi} \cdot \pi r^{3}\)

\(\displaystyle \frac{3}{4\pi} = r^{3}\)

\(\displaystyle r = \sqrt[3]{ \frac{3}{4\pi} } = \sqrt[3]{ \frac{3\cdot 2 \pi^{2}}{4\pi \cdot 2 \pi^{2}} }= \sqrt[3]{ \frac{6 \pi^{2}}{8 \pi^{3}} } = \frac{ \sqrt[3]{6 \pi^{2}} } {2\pi}\) yards. 

 

Since the problem requests the radius in inches, multiply by 36:

\(\displaystyle 36 \times \frac{ \sqrt[3]{6 \pi^{2}} } {2\pi} = \frac{ 18\sqrt[3]{6 \pi^{2}} } {\pi}\)

 

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