问题描述：
The total cost (in dollars) of producing x coffee machines is C(x)=2200+30x−0.8x2.
The total cost (in dollars) of producing x coffee machines is
C(x)=2200+30x−0.8x2.
(A) Find the exact cost of producing the 21st machine.
Exact cost of 21st machine =
(B) Use marginal cost to approximate the cost of producing the 21st machine.
Approx.cost of 21st machine =
The total cost (in dollars) of producing x coffee machines is
C(x)=2200+30x−0.8x2.
(A) Find the exact cost of producing the 21st machine.
Exact cost of 21st machine =
(B) Use marginal cost to approximate the cost of producing the 21st machine.
Approx.cost of 21st machine =
问题解答：
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