问题标题：
英文的导数数学题,(1pt)Thetotalcost(indollars)ofproducingcoffeemachinesisC(x)=1900+70x-0.7x^2(A)Findtheexactcostofproducingthe21stmachine.Exactcostof21stmachine=(B)Usemarginalcosttoapproximatethecostofproduci
问题描述：
英文的导数数学题,
(1pt)Thetotalcost(indollars)ofproducingcoffeemachinesis
C(x)=1900+70x-0.7x^2
(A)Findtheexactcostofproducingthe21stmachine.
Exactcostof21stmachine=
(B)Usemarginalcosttoapproximatethecostofproducingthe21stmachine.
Approx.costof21stmachine=
洪在地回答：
(A)
Exactcostof21stmachine= C(21)-C(20)=3161.3-3120=41.3
(B)
MC(x)=70-1.4x
Approx.costof21stmachine=MC(20)=70-1.4×20=42
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