(4-x的平方)分之(x的平方-2x) =x(x-2)/ -(x+2)(x-2)=-x/(x+2)
通分 （y-x)分之1=2(x+y)(y-x)分之2(x+y)
与（2x+2y）分之1=2(x+y)(y-x)分之y-x