Q4 Given the function y = 4z+9. a. Find the difference quotient as a function of r and Ar b. Find the derivative d Q5 Given the function y = 5x - 4x. a. Find the difference quotient as a function of a and Ar b. Find the derivative d Q6 Given the total cost function C = Q - 5Q +14Q + 75, write out a variable (VC) func- tion. Find the derivative of the VC function and interpret the economic meaning of that derivative. Q7 Given the average-cost function AC = Q - 4Q +214, find the MC function. Is the given function more appropriate as a long-run or a short-run function? Why?