The demand for a certain commodity is D(x) = 4000e-.09x units per month when the market price is x dollars per unit.
(a) At what rate is the consumer expenditure E(x) = xD(x) changing with respect to price x when the price is equal to $120 dollars?
(b) At what price does consumer expenditure stop increasing and begin to decrease?
(c) At what price does the rate of consumer expenditure begin to increase?