Suppose that the demand for x units of a product is
x = 10,000 - 100p
where p dollars is the price per unit. Then the consumer expenditure for the product is
E(p) = px = p(10,000 - 100p) = 10,000p - 100p2
What is the instantaneous rate of change of consumer expenditure with respect to price at
(a) Any price p?
(b) p = 5?
(c) p = 20?