`A=x/y+z/t`
Ta có: $\begin{cases}z\geq y\\x\geq 1\end{cases}⇔xz\geq y⇔\dfrac{x}{y}\geq \dfrac{1}{z}$
`t\le 25⇔z/t\ge z/25`
`⇔A=x/y+z/t\ge 1/z+z/25\ge 2\sqrt[1/z. z/25]=2/5`
Dấu `=` xảy ra $⇔\begin{cases}z=y\\x=1\\t=25\\\dfrac{1}{z}=\dfrac{z}{25}\end{cases}⇔\begin{cases}x=1\\y=z=5\\t=25\end{cases}$
Vậy $Min_A=\dfrac{2}{5}⇔\begin{cases}x=1\\y=z=5\\t=25\end{cases}$
