`3) x^2-xy+2xy-2y^2=5`
`<=>x(x-y)+2y(x-y)=5`
`<=>(x-y)(x+2y)=5`
Do `x,y∈ZZ`
`TH_1`
`=>`\(\left[ \begin{array}{l}\left\{\begin{matrix} x-y=1\\x+2y=5\end{matrix}\right.\\\left\{\begin{matrix} x-y=5\\x+2y=1\end{matrix}\right.\end{array} \right.\)
`<=>`\(\left[ \begin{array}{l}\left\{\begin{matrix} x=7/3\\y=4/3\end{matrix}\right.\\\left\{\begin{matrix} x=11/3\\y=-4/3\end{matrix}\right.\end{array} \right.(ktm)\)
`TH_2`
`=>`\(\left[ \begin{array}{l}\left\{\begin{matrix} x-y=-1\\x+2y=-5\end{matrix}\right.\\\left\{\begin{matrix} x-y=-5\\x+2y=-1\end{matrix}\right.\end{array} \right.\)
`=>`\(\left[ \begin{array}{l}\left\{\begin{matrix} x=-7/3\\y=-4/3\end{matrix}\right.\\\left\{\begin{matrix} x=-11/3\\y=4/3\end{matrix}\right.\end{array} (ktm)\right.\)
