AR (average income): average income

Sir (marginal income): marginal income

Marginal cost: marginal cost

Price: price

AC (average cost): average cost.

AC=AVC+AFC (average fixed cost)

TC=VC+FC

MC = dTC/dQ；

AC = TC/Q；

TR=P*Q

MR=dTR/dQ

AR=TR/Q

The equilibrium condition of all markets is MR=MC.

Perfect competitive market, because the manufacturer is the price receiver, so for perfect competitive market, P = Mr = AR, so the equilibrium condition should be P = MC.