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ACT Math : How to find the radius of a sphere

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Example Questions

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

The surface area of a sphere is $64\pi$ feet. What is the radius?

Possible Answers:

Correct answer:

Explanation:

Solve the equaiton for the surface area of a sphere for the radius and plug in the values:

$SA=4{\pi}r^2\rightarrow r=\sqrt{\frac{SA}{4\pi}}= \sqrt{\frac{64\pi}{4\pi}}=\sqrt16=4$

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Example Question #2 : Spheres

What is the radius of a sphere with a volume of $2304\pi$ ft^3 ? Round to the nearest hundredth.

Possible Answers:

9.53

14.51

4.81

Correct answer:

Explanation:

Recall that the equation for the volume of a sphere is:

$V=\frac{4}{3}\pi r^3$

For our data, we know:

$2304\pi=\frac{4}{3}\pi r^3$

Solve for . First, multiply both sides by $\frac{3}{4}$ :

$\frac{3}{4}2304\pi=\pi r^3$

$1728\pi=\pi r^3$

Now, divide out the $\pi$ :

1728= r^3

Using your calculator, you can solve for . Remember, if need be, you can raise 1728 to the power of $\frac{1}{3}$ if your calculator does not have a variable-root button.

This gives you:

r=12

If you get something like 11.999999.... , just round up. This is a rounding issue with some calculators.

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Example Question #1 : Spheres

The volume of a sphere is $972\pi\:cm^3$ . What is the diameter of the sphere? Round to the nearest hundredth.

Possible Answers:

81.4

21.31

Correct answer:

Explanation:

Recall that the equation for the volume of a sphere is:

$V=\frac{4}{3}\pi r^3$

For our data, we know:

$972\pi = \frac{4}{3}\pi r^3$

Solve for . Begin by dividing out the $\pi$ from both sides:

$972 = \frac{4}{3} r^3$

Next, multiply both sides by $\frac{3}{4}$ :

$\frac{3}{4} * 972 =r^3$

729=r^3

Using your calculator, solve for . Recall that you can always use the $\frac{1}{3}$ power if you don't have a variable-root button.

You should get:

r=9 If you get r=8.99999999... , just round up to . This is a general rounding problem with calculators. Since you are looking for the diameter, you must double this to .

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Example Question #1 : Spheres

What is the radius of a sphere with a surface area of $169\pi$ mm^2 ? Round to the nearest hundredth.

Possible Answers:

16.5

6.5

5.02

8.15

7.15

Correct answer:

6.5

Explanation:

Recall that the surface area of a sphere is found by the equation:

$SA = 4\pi r^2$

For our data, this means:

$169\pi = 4\pi r^2$

Solve for . First, divide by $4\pi$ :

42.25=r^2

Take the square root of both sides:

r = 6.5

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Example Question #5 : How To Find The Radius Of A Sphere

What is the radius of a sphere with a volume of $36\prod \textup{ units}^{3}$ ?

Possible Answers:

$27\textup{ units}$

$12\textup{ units}$

$9\textup{ units}$

$6\textup{ units}$

$3\textup{ units}$

Correct answer:

$3\textup{ units}$

Explanation:

Given the volume of the sphere, $36\prod \textup{units}^{3}$ , you need to use the formula for volume of a sphere $\left ( \left ( \frac{4}{3} \right )\cdot \prod \cdot r^{3}\right )$ and work backwards to find the radius. I would multiply both sides by $\frac{3}{4}$ to get rid of the $\frac{4}{3}$ in the formula. You then have $27\prod=\prod r^{3}$ . Next, divide both sides by $\prod$ so that all vyou have left is $27=r^{3}$ . Finally take the cube root of , to get units for the radius.

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Example Question #6 : How To Find The Radius Of A Sphere

A cube with sides of 25m is circumscribed by a sphere, such that all eight vertices of the cube are tangent to the sphere. What is the sphere's radius?

Possible Answers:

$\frac{125\sqrt5}{2}$

125

$125\sqrt5$

$\frac{25\sqrt{3}}{2}$

$25\sqrt3$

Correct answer:

$\frac{25\sqrt{3}}{2}$

Explanation:

Solving this problem requires recognizing that since the cube is circumscribed by the sphere, both solids share the same center. Now it is just a matter of finding the diagonal of the cube, which will double as the diameter of the sphere (by definition, any straight line which passes through the center of the sphere). The formula for the diagonal of a cube is $D = s\sqrt{3}$ , where is the length of the side of a cube. (This occurs because you must use the Pythagorean theorem once for each 2-dimensional "corner" you travel to find the diagonal for a 3-dimensional shape, but for the ACT it's much faster to memorize the formula.)

In this case:

$D = s\sqrt3 = 25\sqrt3$

Since the radius is half the diameter, divide the result in half:

$r = \frac{D}{2} = \frac{25\sqrt{3}}{2}$

$r = \frac{25\sqrt{3}}{2}$

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ACT Math : How to find the radius of a sphere

Study concepts, example questions & explanations for ACT Math

All ACT Math Resources

Example Questions

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

Example Question #2 : Spheres

Example Question #1 : Spheres

Example Question #1 : Spheres

Example Question #5 : How To Find The Radius Of A Sphere

Example Question #6 : How To Find The Radius Of A Sphere

All ACT Math Resources

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