若y/x,1/y,1/(x+y)都有意义,则下列等式中,不一定成立的是,说明理由
xy/(x^2y)=1/x
1/(x-y)=(x+y)/(x^2-y^2)
y/(x+1)=y^2/(xy+y)
y/(x+y)=(xy-y^2)/(x^2-y^2)