\(C=13x+2012-x^2\)
\(=-\left(x^2-13x+\dfrac{169}{4}\right)+\dfrac{7879}{4}\)
\(=-\left(x-\dfrac{13}{2}\right)^2+\dfrac{7879}{4}\)
Nhận xét :
\(\left(x-\dfrac{13}{2}\right)^2\ge0\)
\(\Leftrightarrow-\left(x-\dfrac{13}{2}\right)^2\le0\)
\(\Leftrightarrow-\left(x-\dfrac{13}{2}\right)+\dfrac{7879}{4}\le\dfrac{7879}{4}\)
\(\Leftrightarrow C\le\dfrac{7879}{4}\)
Dấu "=" xảy ra khi : \(\left(x-\dfrac{13}{2}\right)^2=0\Leftrightarrow x=\dfrac{13}{2}\)
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