Suy ra \(\sqrt{x+1}+\sqrt{7-x}+\sqrt{y+1}+\sqrt{7-y}\le8\) (2)
Từ (1) và (2) suy ra \(\left\{{}\begin{matrix}\dfrac{\sqrt{x+1}}{1}=\dfrac{\sqrt{7-x}}{1}\\\dfrac{\sqrt{y+1}}{1}=\dfrac{\sqrt{7-y}}{1}\end{matrix}\right.\) \(\Leftrightarrow\) \(\left\{{}\begin{matrix}x=3\\y=3\end{matrix}\right.\) thỏa mãn