the volume V of a right circular cone of height h=8 feet and radius r feet is V=V(r)=2pier^2. If r is changing, find the instantaneous rate of change of the volume V with respect to the radius r at r=4.

Answer 1

PI, not pie!
It's a Greek letter, not a dessert.

iv v = 2πr^2,
dv/dr = 4πr
now just plug in r=4

However, for a cone of height 8,
v = 8/3 πr^2
If that's what you want, make the necessary change to the above logic.