`x^2 - 4y^2 = 3xy`
`tox^2 - 4y^2 - 3xy=0`
`to(x^2 - y^2)-(3y^2+ 3xy)=0`
`to(x- y)(x+y)-3y(y+ x)=0`
`to(x+ y)(x-4y)=0`
\(\to\left[ \begin{array}{l}x=-y\\x=4y\end{array} \right.\)
`text(nếu )x=-y`
`A = (-y-y)/(-y+y)=(-2y)/0(text(vô lí))`
`text(nếu )x=4y`
`A = (4y-y)/(4y+y)=(3y)/(5y)=3/5`