Posted by Anonymous on .
The function V=4pir^2 describes the volume of a right circular cylinder of heigh 4ft and radius r feet. Find the (instantaneous) rate of change of the volume with respect to the radius when r=6. Leave the answer in terms of pi.

Calculus 
Reiny,
dV/dr = 8pir*dr/dr
which at r = 6 is 8pi(6)
=48pi 
Calculus (for reiny) 
Anonymous,
actually the answer is 64pi ft^3/ft.
can you figure out what you did wrong? thanks. 
Calculus (for reiny) 
Reiny,
Nope, for the data you gave me the answer is 48pi
this is a very straighforward question,
to have an answer of 64 pi, perhaps you were to find the rate when r = 8.
Check your typing 
Calculus 
Anonymous,
sorry. yes, r should be equal to 8. Thanks.