Answers

2016-04-14T13:22:15+05:30
TC = 3 q² + 7 q + 12

average cost function = TC / q = 3 q + 7  + 12/q
   for q = 3,  it is  20

marginal cost function =  d (TC)/dq = 6 q + 7 
       for q = 3,  it is  25..
   Actually this method is quite good for large values of q.

marginal cost function in another way: 
 MC(q) = TC(q) - TC(q-1) =  3[q² - (q-1)²] + 7 [q - (q-1)]
           = 6 q - 3 + 7  =  6 q + 4

   MC(3) = 22
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