a)\(A=\dfrac{7\sqrt{x}+9}{\sqrt{x}+1}\left(đk:x\ge0\right)=\dfrac{7\left(\sqrt{x}+1\right)}{\sqrt{x}+1}+\dfrac{2}{\sqrt{x}+1}=7+\dfrac{2}{\sqrt{x}+1}\le7+\dfrac{2}{0+1}=9\)
\(maxA=9\Leftrightarrow x=0\)
b) \(A=\dfrac{-2\sqrt{x}-1}{2\sqrt{x}+3}\left(đk:x\ge0\right)=\dfrac{-\left(2\sqrt{x}+3\right)}{2\sqrt{x}+3}+\dfrac{2}{2\sqrt{x}+3}=-1+\dfrac{2}{2\sqrt{x}+3}\le-1+\dfrac{2}{0+3}=-\dfrac{1}{3}\)
\(maxA=-\dfrac{1}{3}\Leftrightarrow x=0\)
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