The cost, in dollars, of producing x belts is given by C(x) = 794 + 14x - 0.073x". Find the rate at which average cost is changing when 400 belts have been produced. When 400 belts have been produced, the average cost is changing at for each additional belt. (Round to four decimal places as needed. Do not include the $ symbol in your answer.)The population of a city grows from an initial size of 400,000 to a size P given by Pit) = 400,000 + 7000?, where t is in years. dP a] Find the growth rate, E" b) Find the population after 20 yr. 1:] Find the growth rate it-=20. <: dp a find the growth rate e.>