\(A=x-2\sqrt{x}\left(\sqrt{y}+1\right)+\left(\sqrt{y}+1\right)^2-\left(\sqrt{y+1}\right)^2+3y+1\)
\(A=\left(\sqrt{x}-\sqrt{y}-1\right)^2-\left(y+2\sqrt{y}+1\right)+3y+1\)
\(A=\left(\sqrt{x}-\sqrt{y}-1\right)^2+2y-2\sqrt{y}\)
\(A=\left(\sqrt{x}-\sqrt{y}-1\right)^2+2\left(y-2.\sqrt{y}.\frac{1}{2}+\frac{1}{4}\right)-\frac{1}{2}\)
\(A=\left(\sqrt{x}-\sqrt{y}-1\right)^2+2\left(\sqrt{y}-\frac{1}{2}\right)^2-\frac{1}{2}\ge-\frac{1}{2}\forall x,y\ge0\)
Dấu "="\(\Leftrightarrow\hept{\begin{cases}\sqrt{x}-\sqrt{y}-1=0\\\sqrt{y}=\frac{1}{2}\end{cases}\Leftrightarrow\hept{\begin{cases}x=\frac{9}{4}\\y=\frac{1}{4}\end{cases}}}\)
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