If f and g are functions of time, and at time t = 3, f equals 8 and is rising at a rate of 3 units per second, and g equals 5 and is rising at a rate of 8 units per second, then the product fg equals __ and is rising at a rate of __ units per second.

Question

I got 40 for the first answer, but need help with the second please!

Answer 1

I'm sure your text includes the derivation of the formula that the rate of change of fg is

∆(fg)/∆t = f * ∆g/∆t + g * ∆f/∆t
In this case, the rate of change of fg is thus

8*5 + 5*3 = 55 units/sec