分子=x^2-3xy+2y^2=（x-y)(x-2y)
分母=-4x^2y^2+x^3y+4xy^3=xy(x^2-4xy+4y^2)=xy(x-2y)^2
所以,原式=[(x-y)(x-2y)]/[xy(x-2y)^2]=(x-y)/[xy(x-2y)]