x²+3xy-4y²=0
(x+4y)(x-y)=0
∴x=-4y 或 x=y
当x=-4y时 (x-y)/(x+y)=(-4y-y)/(-4y+y)=5/3
当x=y时 (x-y)/(x+y)=(y-y)/(y+y)=0