Derivative of a right circular cone

[b.mueller5]

inverted right-circular cone, with radius at the top 15 meters and depth 12 meters. rate of 2 cublic meters per minute. How fast is the depth increasing at the instant when the depth is 8 meters.

When i try to solve it I get an equation with 2 varibles.

 

[grzz]

Note that one can relate radius r at any point to the depth h at that point. Then dV/dt will be in terms of dh/dt only.

 

[HallsofIvy]

Think of one side of the cone as starting at (0, 0) and going to (15, 12). What is the equation of that line?