I've got one problem left on a math assignment that I can't figure out. I don't usually post stuff like this here, but this one really confuses me.

"A cylindrical drum with a radius of 1.5 metres is being filled with oil"

Part A: "Find the formula to represent the instantaneous rate of change in the volume of oil with respect to the height of water".

So far I just have V(h)=pi*r^2*h. The formula for the volume of a cylinder.

We're doing derivatives and stuff, so I have to (I'm pretty sure), find the derivative of that function to be able to get the IROC.

I thought the derivative would be V(h)'=pi*2r*h, but apparently that's not right since the radius is constant. But the only other way I can see to do it is if V(h)'=pi*r^2. But that doesn't actually include height anywhere in the function.

Is this something I'm just missing in the derivative, or something in the problem itself?

Thanks.