With regard to the question, the equation representing the marginal cost MC ought to represent the total cost TC.
Therefore
TC=10−2Q+2Q2
at optimal profit the marginal cost equals marginal revenue equal market price
MC=MR=MP
MC=dQdTC=−2+4Q=8
4Q=10
output is
Q=25or2.5
TC=10−2(25)+2(25)2
TC=17.5
Total Revenues = Quantity * price
TR=25∗8=20
Profit = TR -TC
Profit =20−17.5=2.5
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