# Math Help - Related Rates [Balloon]

1. ## Related Rates [Balloon]

The radius of a speherical balloon is decreasing at a rate of 3.5 cm/s. At what rate is the surface area of the balloon decreasing when the radius is 14cm?

dr/dt = -3.5 cm/s

da/dt = ? when r = 14

Ive used the area of a sphere forumla, derived, plugged in numbers but i'm getting a wrong answer.

2. Originally Posted by Raj
Ive used the area of a sphere forumla, derived, plugged in numbers but i'm getting a wrong answer.
Could you show us your work? We can point out your mistake.

3. Originally Posted by Raj
The radius of a speherical balloon is decreasing at a rate of 3.5 cm/s. At what rate is the surface area of the balloon decreasing when the radius is 14cm?

dr/dt = -3.5 cm/s

da/dt = ? when r = 14

Ive used the area of a sphere forumla, derived, plugged in numbers but i'm getting a wrong answer.

Your set up is correct .

$\frac{dA}{dr}=4\pi r^2$

$
=8\pi r=112\pi
$

just multiply it with -3.5 to get the answer .