1. Consider the following short- run variable cost function for
a firm.
VC = q3 - 10q2 + 100q
a. Find the equation for the short-run average variable cost and
marginal cost.
b. Using your answer to part a, show that the output label at which
short-run average variable cost is minimized is greater than the
output level at which short-run marginal cost is minimized.
c. Using your answers to part (a) and (b), draw a rough sketch of
the short run marginal and average cost curves.
d. Using your answers to part c, show the price label at which the
firm would shut down and how for any one price label at which the
firm earns a profit.
VC = q3 - 10q2 + 100q
(a)
AVC = VC/q = q2 - 10q + 100
MC = dVC/dq = 3q2 - 20q + 100
(b)
AVC is minimized when dAVC/dq = 0
dAVC/dq = 2q - 10 = 0
2q = 10
q = 5
MC is minimized when dMC/dq = 0
dMC/dq = 6q - 20 = 0
6q = 20
q = 3.33 < 5 (Proved)
(c)
(d)
Firm will shut down when Price < Minimum AVC. So, shut-down price = Minimum AVC
AVC is minimum when q = 5. At this level,
Shut-down price = Minimum AVC = (5 x 5) - (10 x 5) + 100 = 25 - 50 + 100 = 75
In above graph, P* is the shut-down point. When Price is higher than P*, Price > AVC and therefore firm makes a profit.
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