Question

Given total cost TC=3q²+7q=12, evaluate the marginal cost and average cost if q=3.

Answer 1

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
 

Answer 2

Explanation:

hope it is helpful ✌️✌️