The demand, D, for a new rollerball pen is given by D = 0.007p3 0.6p2 + 120p, where p is the price in dollars.
a. Find the rate of change of quantity with respect to price, dD/dp.
b. How many units will consumers want to buy when the price is $25 per unit?
c. Find the rate of change at p = 25, and interpret this result.
d. Would you expect dD/dp to be positive or negative?