（x√x+x√y)÷(xy-y^2)--(x+√xy+y)÷(x√x-y√y)
=x(√x+√y)/[y(x-y)]-[x+√(xy)+y]/[(√x)³-(√y)³]
=x(√x+√y)/[y(√x+√y)(√x-√y)]-[x+√(xy)+y]/[(√x-√y)(x+√(xy)+y)]
=x/[y(√x-√y)]-1/(√x-√y)
=(x-y)/[y(√x-√y)]
=(√x+√y)(√x-√y)/[y(√x-√y)]
=(√x+√y)/y.