Each of Exercises 61 through 68 involves either the chain rule for partial derivatives or the incremental approximation formula for functions of two variables.

A car dealer determines that if gasoline-electric hybrid automobiles are sold for x dollars apiece and the price of gasoline is y cents per gallon, then approximately H hybrid cars will be sold each year, where H(x, y) = 3,500 − 19x^1/2 + 6(0.1y + 16)^3/2 She estimates that t years from now, the hybrid cars will be selling for x(t) = 35,050 + 350t dollars apiece and that gasoline will cost y(t) = 300 + 10(3t)1/2 cents per gallon. At what rate will the annual demand for hybrid cars be changing with respect to time 3 years from now? Will it be increasing or decreasing?