The cost C of building a house is related to the number k of carpenters used and the number x of electricians used by the following formula. + C = 15,000 + 50k2 + 60x2 (a) Assuming that 10 carpenters are currently being used, find the cost function C, marginal cost function C', and average cost function C, all as functions of x. C(X) = C'(x) = C(X) = (b) Use the functions you obtained in part (a) to compute C'(13) and C(13). (Round your answers to the nearest cent.) C'(13) = $ per spot C(13) = $ per spot Use these two answers to say whether the average cost is increasing or decreasing as the number of electricians increases. The average cost is increasing as x increases. The average cost is decreasing as x increases