Please help with this

Question

The volume of a cylinder of height 9 inches and radius rinches is given by the formula V=9πr2.

Which is the correct expression for dVdt?

1. dVdt=0
2. dVdt=18πrdrdtdhdt
3. dVdt=18πrdt
4. dVdt=9πr2drdt
5. dVdt=18πrdrdt

Correct

Suppose that the radius is expanding at a rate of 0.2 inches per second. How fast is the volume changing when the radius is 2.8 inches

Answer 1

For the last portion, dr/dt = 0.2 (a positive value since it is expanding) with r = 2.8 so just substitute those values in.

Answer 2

last option, choice number 5

dV/dt = 18pi(2.8)(.2)

dV= 18pirdr/dt

replace r with 2.8

replace dr/dt with .2

if you want, replace pi with 3.14