Example
The surface area of the sphere pictured on the left is:
$$ A = 4\pi \cdot r^2 \\ A = 4\pi \cdot 3^2 \\ A = 36\pi \\ = 113.1 $$
Practice Problems
Problem 1
First, remember that we need to divide the diameter by 2 to get the radius of 6 for our formula:
$$ A = 4\pi \cdot 6^2 \\ A = 4\pi \cdot 6^2 \\ A = 144\pi \\ = 452.4 cm^2 $$
Problem 2
Substitute the known values into the formula and solve for the radius.
$$ A = 4\pi \cdot r^2 \\ 196\pi = 4\pi \cdot r^2 \\ \frac{196\pi}{4\pi} = \frac{4\pi \cdot r^2}{4\pi} \\ 49 = r^2 \\ r^2 = \sqrt{49} = 7\text{ inches} $$
Related Links:
