Pove that if a revenue function rx is concave downward rx

Question

Question: MAXIMIZING REVENUE The average revenue is defined as the function

R¯(x) = R(x)/x          (x > 0)

Prove that if a revenue function R(x) is concave downward [R"(x) = 0], then the level of sales that will result in the largest average revenue occurs when R(x) = R'(x).