Question

a spherical balloon has a circumference of 21 cm. what is the approximate surface area of the balloon to the nearest square centimeter?

Answer 1

use the circumference of the sphere to find the radius

\[21 = 2\times \pi \times r\]

when you get a value substitute it into the surface area formula

\[SA = 4 \pi r^2\]

and alternate method to use

\[SA = \frac{4 \pi^2 r^2}{\pi} \]

which is

\[SA = \frac{(2\pi r)^2}{\pi}\]

or

\[SA = \frac{C^2}{\pi}\]