Giải:
\(C=x+1-x^2\)
\(\Leftrightarrow C=x+\dfrac{5}{4}-\dfrac{1}{4}-x^2\)
\(\Leftrightarrow C=\dfrac{5}{4}+x-\dfrac{1}{4}-x^2\)
\(\Leftrightarrow C=\dfrac{5}{4}-\left(-x+\dfrac{1}{4}+x^2\right)\)
\(\Leftrightarrow C=\dfrac{5}{4}-\left(x-\dfrac{1}{2}\right)^2\le\dfrac{5}{4}\)
\(\Leftrightarrow C_{Max}=\dfrac{5}{4}\)
\(\Leftrightarrow x-\dfrac{1}{2}=0\Leftrightarrow x=\dfrac{1}{2}\)
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