Surface area (S) of a cylinder:
Since you are dealing with the diameter, it would be nice to express the radius in terms of its diameter (D):
Differentiate with respect to time:
And solve for
A 100ft. long cable of diameter 4 inches is submerged in seawater. Due to corrosion, the surface area of the cable decreases at a rate of 750 in^2 / year. Ignoring the corrosion at the ends of the cable, find the rate at which the diameter is decreasing.