2x²-3xy-2y²=0
(2x+y)(x-2y)=0
x=2y,或x=-y/2
所以
（x+y）/（x-y）
=（2y+y)/(2y-y)
=3
或=（-y/2+y)/(-y/2-2y)
=(y/2)/(-5y/2)
=-1/5