[(x/x-y) -1]/(y的平方/x+y)乘以(x-x的平方/x+y)
=(x-x+y)/(x-y) ×(x+y)/y²×(x²+xy-x²)/(x+y)
=y/(x-y) ×(x+y)/y²×xy/(x+y)
=x/(x-y)

,当x=—2,y=—1.时
原式=-2/(-2+1)=2