The demand for a certain commodity is

D(x) = 500 e^-0.2x

units per month when the market price isxdollars per unit.

(a)At what rate is the consumer expenditureE(x) =xD(x) changing with respect to pricexwhen the price is equal to $60 dollars?

(b)At what price does consumer expenditure stop increasing and begin to decrease?(c)At what price does therateof consumer expenditure begin to increase?