复旦大学高级微观经济学讲义 黄有光 张欣

VI. Firm’s Supply: the short run and long run (IV)

6.1  Additional constraints to the profit maximization problem

       
6.2 The short-run
•        In the short run, some inputs are fixed, called fixed inputs
•        Others are variable inputs
•        the cost associated with the fixed inputs is the fixed cost
  F  Fixed costs
•        the cost associated with the variable inputs is the variable cost
  Cv (y)  Variable costs
•        Total cost:  C = F + Cv(y)
•        The short run decision:

•        Remarks:
a)  The intercept of C(y) is the fixed cost
b)  MC = C’(y) = Cv’(y)
c)  ATC  average total cost  C(y) / y
                 AVC  average variable cost  Cv(y) / (y)
             The marginal cost goes through the minimum of the ATC and AVC.
             Proof
•        Example: Given the cost function  C = y3 – 4y2 + 6y + 10, find the supply function.

6.3  The long run
        In the long run, all inputs are variable.  No fixed cost.
•        The long run supply curve versus the short run supply curves

6.4  The Producers’ Surplus
•        The producer’s surplus (PS) is defined as the “triangle area” between the price line and the supply curve (MC).
•        An alternative definition is   PS = P y – Cv(y)
•        Prove that two definitions are the consistent.
•        Economic implication of the producers’ surplus


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