Recall that if the cost is C ( q ) {\displaystyle C(q)} as a function of the quantity q {\displaystyle q} , then the average cost is C ( q ) q {\displaystyle {\frac {C(q)}{q}}} and the marginal cost is C ′ ( q ) = d C d q ( q ) {\displaystyle C'(q)={\frac {dC}{dq}}(q)} .