Question

Water is flowing freely from the bottom of a conical tank which is 12 feet deep and 6 feet in radius at the
top. If the water is flowing at a rate of 2 cubic feet per hour, at what rate is the depth of the water in the
tank going down when the depth is 3 feet.

Answer 1

When the water is x feet deep, and the surface has radius r, then by similar triangles,

x/r = 12/6
x = 2r
r = x/2

So, since the volume of water

v = 1/3 pi r^2 x
= 1/12 pi x^3

dv/dt = pi/4 x^2 dx/dt

-2 = pi/4 (9) dx/dt

dx/dt = -8/(9pi) = -.28 ft^3/hr