[tex]\left\{ \begin{array}{l} \sqrt{x-y} = 9-|x+2y| \\ x(x+4y-2)+y(4y+2)=41 \end{array} \right.[/tex]
[TEX]HPT \Leftrightarrow \left{\begin{\sqrt{x-y}=9-|x+2y|}\\{x^2+4xy-2x+4y^2+2y=41}[/TEX]
[TEX]\Leftrightarrow \left{\begin{\sqrt{x-y}=9-|x+2y|
}\\{(x+2y)^2-2(x-y)=41}[/TEX]
Đặt [TEX]\left{\begin{a=\sqrt{x-y} \geq 0}\\{b=|x+2y|}[/TEX]
Ta được HPT:
[TEX]\left{\begin{a=9-b}\\{b^2-2a=41}[/TEX]
Dùng PP thế là OK rồi