-(16x^2 y^3)/(20xy^4)=-4x/(5y)
.
(x^2-9)/（x-3）=x+3
.
(x^2-2x)/(2y-xy)=x(x-2)/[y(2-x)]=-x/y
.
(m^2-2m+1)/(1-m^2）=(m-1)²/[(1-m)(1+m)]=(1-m)/(1+m)