pt(2) \Leftrightarrow xy= x^2-5x+8
thay vao pt (1) ta duoc:
x^2-2(x^2-5x+8)+y^2+4y-5=0
\Leftrightarrow x^2-2x^2+10x-16-5+y^2+4y=0
\Leftrightarrow (y^2+4y+4)-(x^2-10x+25)=0
\Leftrightarrow (y+2)^2-(x-5)^2=0
\Leftrightarrow (y-x+7)(y+x-3)=0
sau do rút x hoac y ra roi the vao pt (2) la dk