Observe that in th example c"(x) < 0 when x < 100. This mean when x < 100, increase in production lead to decrease in marginal cost. The graph of c'(x) as a function of x slopes downwards when x < 100. However,when x > 100, the graph of c'(x) slopes upwards,so c" is positive. In this case, increase in production lead to increase in the marginal