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

Question

campbell_st · Answer

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 

$$SA = \frac{C^2}{\pi}$$