GRE Math : How to find the surface area of a sphere

Study concepts, example questions & explanations for GRE Math

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Example Questions

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

Find the surface area of a sphere with a diameter of 14. Use π = 22/7.

Possible Answers:

616

872

1256

428

2464

Correct answer:

616

Explanation:

Surface Area = 4πr2 = 4 * 22/7 * 72 = 616

Example Question #1531 : Gre Quantitative Reasoning

A sphere has a surface area of \displaystyle 16\pi square inches.  If the radius is doubled, what is the surface area of the larger sphere?

Possible Answers:

\displaystyle 48\pi \ in^2

Cannot be determined

\displaystyle 64\pi \ in^2

\displaystyle 32\pi \ in^2

\displaystyle 16\pi \ in^2

Correct answer:

\displaystyle 64\pi \ in^2

Explanation:

The surface area of the larger sphere is NOT merely doubled from the smaller sphere, so we cannot double \displaystyle 16\pito find the answer.

We can use the surface area formula to find the radius of the original sphere.

\displaystyle 4\pi r^2=16\pi

r2 = 4

r = 2

Therefore the larger sphere has a radius of 2 * 2 = 4.

The new surface area is then \displaystyle 4\pi \times 4^2=64\pi square inches.

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

If a sphere has a volume of \displaystyle 36\pi cubic inches, what is its surface area?

Possible Answers:

\displaystyle 96\pi\ in^2 

\displaystyle 24\pi\ in^2 

\displaystyle 36\pi \ in^2 

\displaystyle 32\pi\ in^2 

\displaystyle 108\pi\ in^2 

Correct answer:

\displaystyle 36\pi \ in^2 

Explanation:

The volume of a cube is equal to \displaystyle v=\frac{4}{3}\pi r^3.

So we mutiply our volume by \displaystyle \frac{3}{4} and divide by \displaystyle \pi, giving us \displaystyle r=3.

The surface area of a sphere is equal to \displaystyle 4\pi r^2, giving us \displaystyle 36\pi.

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