（1）8x^2y^2*(-3x/4y^5)
=[8x^2y^2*(-3x)*y^-5]/4
=-6x^3y^-3
=-6x^3/y^3
(2)(x+y)^2/(xy-y^2)*[(y-x)^2/(xy+y^2)]
=[(x+y)^2/y(x-y)]*[(x-y)^2/y(x+y)]
=(x+y)(x-y)/y^2
=(x^2-y^2)/y^2
=-1+x^2/y^2