High School Math : How to find the radius of a sphere

Study concepts, example questions & explanations for High School Math

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Example Questions

Example Question #2011 : High School Math

Find the radius of a sphere whose surface area is \(\displaystyle SA=100cm^{2}\).

Possible Answers:

\(\displaystyle 2.82cm\)

Not enough information to solve

\(\displaystyle 3.54cm\)

\(\displaystyle 35.45cm\)

\(\displaystyle 8.22cm\)

Correct answer:

\(\displaystyle 2.82cm\)

Explanation:

We know that the surface area of the spere is \(\displaystyle SA=100cm^{2}\).

\(\displaystyle SA=4\pi r^{2}\)

\(\displaystyle 100cm^{2}=4\pi r^{2}\)

Rearrange and solve for \(\displaystyle r\).

\(\displaystyle r^{2}=\frac{100cm^{2}}{4\pi}\)

\(\displaystyle \dpi{100} r=\sqrt{\frac{100cm^{2}}{4\pi}}\)

\(\displaystyle \dpi{100}\rightarrow 2.82cm\)

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

What is the radius of a sphere that has a surface area of \(\displaystyle 16\pi\)?

Possible Answers:

\(\displaystyle 2\)

\(\displaystyle 2\sqrt{\pi }\)

\(\displaystyle 8\sqrt{\pi }\)

\(\displaystyle 8\pi\)

\(\displaystyle \sqrt{2\pi }\)

Correct answer:

\(\displaystyle 2\)

Explanation:

The standard equation to find the area of a sphere is \(\displaystyle SA=4\pi r^2\) where \(\displaystyle r\) denotes the radius. Rearrange this equation in terms of \(\displaystyle r\):

\(\displaystyle r=\sqrt{\frac{SA}{4\pi }}\)

To find the answer, substitute the given surface area into this equation and solve for the radius:

\(\displaystyle r=\sqrt{\frac{16\pi }{4\pi }}=\sqrt{4}=2\)

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

Given that the volume of a sphere is \(\displaystyle 12\pi\), what is the radius?

Possible Answers:

\(\displaystyle 9\pi\)

\(\displaystyle \sqrt[3]{12 }\)

\(\displaystyle 3\)

\(\displaystyle \sqrt[3]{9}\)

\(\displaystyle \sqrt{9\pi }\)

Correct answer:

\(\displaystyle \sqrt[3]{9}\)

Explanation:

The standard equation to find the volume of a sphere is 

\(\displaystyle V=\frac{4}{3}\pi r^3\) where \(\displaystyle r\) denotes the radius. Rearrange this equation in terms of \(\displaystyle r\):

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

Substitute the given volume into this equation and solve for the radius:

\(\displaystyle r=\sqrt[3]{\frac{3V}{4\pi }}=\sqrt[3]{\frac{3\cdot 12\pi }{4\pi }}=\sqrt[3]{\frac{36\pi }{4\pi }}=\sqrt[3]{9 }\)

Example Question #31 : Spheres

What is the radius of a sphere with a volume of \(\displaystyle 288\pi\)?

Possible Answers:

\(\displaystyle 12\)

\(\displaystyle 24\)

\(\displaystyle 36\)

\(\displaystyle 6\)

Correct answer:

\(\displaystyle 6\)

Explanation:

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

\(\displaystyle 288\pi=\frac{4}{3}\pi r^3\)

\(\displaystyle \frac{288\pi}{\pi}=\frac{4\pi r^3}{3\pi}\)

\(\displaystyle 288=\frac{4r^3}{3}\)

\(\displaystyle (288)(\frac{3}{4})=\frac{3}{4}(\frac{4r^3}{3})\)

\(\displaystyle 216=r^3\)

\(\displaystyle \sqrt[3]{216}=\sqrt[3]{r^3}\)

\(\displaystyle 6=r\)

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