Bài 1: \(H=4\sqrt{x}-x-y+6\sqrt{y}-15=-\left(x-4\sqrt{x}+\text{4 }\right)+4-\left(y-6\sqrt{y}+9\right)+9-15=-\left(\sqrt{x}-2\right)^2-\left(\sqrt{y}-3\right)^2-2\le-2\)
Vậy H đạt gtln bằng -2 \(\Leftrightarrow\left\{{}\begin{matrix}x=4\\y=9\end{matrix}\right.\)
Bài 2:
(+) \(F=\dfrac{2\sqrt{x}-5}{\sqrt{x}-4}=2-\dfrac{1}{\sqrt{x}-4}\)
\(F\in Z\Leftrightarrow\dfrac{1}{\sqrt{x}-4}\Leftrightarrow\left[{}\begin{matrix}\sqrt{x}-4=-1\\\sqrt{x}-4=1\end{matrix}\right.\)
\(\Leftrightarrow\left[{}\begin{matrix}x=9\left(N\right)\\x=25\left(N\right)\end{matrix}\right.\)
Kl: x=9, x=25
(+) \(G=\dfrac{4\sqrt{x}-9}{\sqrt{x}+4}=\dfrac{4\left(\sqrt{x}+4\right)-16-9}{\sqrt{x}+4}=4-\dfrac{25}{\sqrt{x}+4}\)
\(G\in Z\Leftrightarrow\dfrac{37}{\sqrt{x}+4}\in Z\Leftrightarrow\) (tự làm tiếp nhé)
