最简公分母x(x+1))(x-1)^2
x+1/x(x-1)=x(x-1)(x+1)^2/x(x+1))(x-1)^2
x/x^2-1=x^2(x-1)/x(x+1))(x-1)^2
2/x^2-2x+1=2x(x+1)/x(x+1))(x-1)^2