GRE Math : How to find the radius of a sphere

Study concepts, example questions & explanations for GRE Math

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Example Questions

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

If a sphere has a volume of \(\displaystyle 268.08\) cubic inches, what is the approximate radius of the sphere?

Possible Answers:

\(\displaystyle 5.9 in\)

\(\displaystyle 8 in\)

\(\displaystyle 64 in\)

\(\displaystyle 4.5 in\)

\(\displaystyle 4 in\)

Correct answer:

\(\displaystyle 4 in\)

Explanation:

The formula for the volume of a sphere is 

\(\displaystyle v=\frac{4}{3}(\pi r^3)\) where \(\displaystyle r\) is the radius of the sphere.

Therefore, 

\(\displaystyle r=\sqrt[3]{\frac{3v}{(4\pi)}}\)

\(\displaystyle r=\sqrt[3]{\frac{3(268.08)}{(4\pi)}}=\sqrt[3]{63.999}=3.999\), giving us \(\displaystyle r\approx4\).

Example Question #1541 : Gre Quantitative Reasoning

A rectangular prism has the dimensions \(\displaystyle 9\textup{ x }6\textup{ x }12\). What is the volume of the largest possible sphere that could fit within this solid?

Possible Answers:

\(\displaystyle 288\pi\)

\(\displaystyle 2304\pi\)

\(\displaystyle 36\pi\)

\(\displaystyle 972\pi\)

Correct answer:

\(\displaystyle 36\pi\)

Explanation:

For a sphere to fit into the rectangular prism, its dimensions are constrained by the prism's smallest side, which forms its diameter. Therefore, the largest sphere will have a diameter of \(\displaystyle 6\), and a radius of \(\displaystyle 3\).

The volume of a sphere is given as:

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

And thus the volume of the largest possible sphere to fit into this prism is

\(\displaystyle \frac{4}{3}\pi (3)^3=36\pi\)

Example Question #2 : How To Find The Radius Of A Sphere

What is the radius of a sphere with volume \(\displaystyle 36\pi\) cubed units?

Possible Answers:

\(\displaystyle 4\)

\(\displaystyle 6\)

\(\displaystyle 7\)

\(\displaystyle 2\)

\(\displaystyle 3\)

Correct answer:

\(\displaystyle 3\)

Explanation:

The volume of a sphere is represented by the equation \(\displaystyle \frac{4}{3}\pi r^3\). Set this equation equal to the volume given and solve for r:

\(\displaystyle 36\pi=\frac{4}{3}\pi r^3\)

\(\displaystyle 36\pi\cdot \frac{3}{4}=\pi r^3\)

\(\displaystyle 27\pi=\pi r^3\)

\(\displaystyle 27=r^3\)

\(\displaystyle r=3\)

Therefore, the radius of the sphere is 3.

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