Sand is falling at the rate 27 cubic feet per minute onto a conical pile whose radius is always equal to its height. how fast is the height of the pile growing when the height is exactly (a) 3 feet (b) 6 feet (c) 9 feet.

Step-by-step explanation:

The formula for the volume of a cone is V = (1/3)(area of base)(height).  If the radius is always equal to the height of the cone, then V = (1/3)(πh²)(h), where we have eliminated r.  Shortened, this comes out to V = (1/3)(π)(h³).  

We want to know how fast h is increasing when h = 3 ft.

Taking the derivative dV/dt, we get dV/dt = (1/3)π(3h²)(dh/dt), or, in simpler terms, dV/dt = πh²(dh/dt).  Set this derivative = to 27 ft³/min and set h = 3 ft.

Then 27 ft³/min = π(3 ft)²(dh/dt) and solve for dh/dt:  (3/π) ft/min = dh/dt when h = 3 ft.

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