(xy^2-2xy+x)乘以(y^3-y)分之(y^3+y^2)
=x(y^2-2y+1)*[y^2(y+1)]/[y^2(y-1)]
=x(y-1)^2*[y^2(y+1)]/[y^2(y-1)]
=x(y-1)(y+1)