\(B=6\sqrt{x}-x+2\)
\(B=-\left(x-6\sqrt{x}-2\right)\)
\(B=-\left(x-2\cdot\sqrt{x}\cdot3+9-11\right)\)
\(B=-\left[\left(\sqrt{x}-3\right)^2-11\right]\)
\(B=11-\left(\sqrt{x}-3\right)^2\le11\forall x\)
Dấu "=" xảy ra \(\Leftrightarrow\sqrt{x}=3\Leftrightarrow x=9\)
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\(C=-x+\sqrt{x+3}\)
\(C=-\left(x-\sqrt{x+3}\right)\)
\(C=-\left(x+3-2\sqrt{x+3}\cdot\frac{1}{2}+\frac{1}{4}-\frac{1}{4}-3\right)\)
\(C=-\left[\left(\sqrt{x+3}-\frac{1}{2}\right)^2-\frac{13}{4}\right]\)
\(C=\frac{13}{4}-\left(\sqrt{x+3}-\frac{1}{2}\right)^2\le\frac{13}{4}\forall x\)
Dấu "=" xảy ra \(\Leftrightarrow\sqrt{x+3}=\frac{1}{2}\Leftrightarrow x+3=\frac{1}{4}\Leftrightarrow x=\frac{-11}{4}\)( thỏa )