13 Given a total-revenue function R(x) = 800x -0.2x and a total-cost function C(x) = 1600 (x + 5) 3 + 600, both in thousands of dollars, find the rate at which total profit is changing when x items have been produced and sold. P'(x) = An accessories company finds that the cost, in dollars, of producing x belts is given by C(x) = 780 + 40x-0.062x. Find the rate at which average cost is changing when 172 belts have been produced. First, find the rate at which the average cost is changing when x belts have been produced. C'(x) = An accessories company finds that the cost and revenue, in dollars, of producing x belts is given by C(x)=730+38x-0.069x2 and R(x) = 55x 10, respectively. Determine the rate at which the accessories company's average profit per belt is changing when 174 belts have been produced and sold. First, find the rate at which the average profit is changing when x belts have been produced. P'(x)=