问一道化简求值题

[y/(x-y)]+[y^3/(x^3-2x^2y+xy^2)]/[(xy+y^2)/(y^2-x^2)]
=[y/(x-y)]+[y^3/x(x-y)^2]/[y(x+y)/(y-x)(y+x)]
=[y/(x-y)]+[y^3/x(x-y)^2]*[(y-x)/y]
=[y/(x-y)]+[y^2/x(y-x)]
=[xy/x(x-y)]-[y^2/x(x-y)]
=(xy-y^2)/x(x-y)
=y(x-y)/x(x-y)
=y/x
=-3/(2/3)
=-3*(3/2)
=-9/2.

解：[y/(x-y)]+[y^3/(x^3-2x^2y+xy^2)]/[(xy+y^2)/(y^2-x^2)]
=[y/(x-y)]+[y^3/x*(x^2-2xy+y^2)]/[y*(x+y)/(y-x)*(y+x)]
=[y/(x-y)]+[y^3/x*(x-y)^2]/[y/(y-x)]
=[y/(x-y)]+[y^2/x*(y-x)]
=[x*y/x*(x-y)]-[y^2/x*(x-y)]
=y*(x-y)/x*(x-y)
=y/x
将x=2/3,y=-3代入上式
原式=-9/2