請問一個個體經濟觀念 - 經濟

Firm X's average total cost per month equals 5Q,where Q is the number of units of output produced per month.The marginal cost of the third unit

produced per month is ?

答案是30

我有個問題:

total cost=5Q^2

所以MC=10Q

如果帶入Q=3到MC-->MC=30得解

可是如果我想從總成本看

不就是5(3^2-2^2)=25嗎

請問問題出在哪??

照理說不是兩者應該相等嗎????

謝謝回答囉~
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