原式1/(x-1)+(x^2-3x)/(x+1)(x-1),然后通分得 (x+1+x^2-3x)/(x+1)(x-1)=(x^2-2x+1)/(x+1)(x-1)=(x-1)(x-1)/(x-1)(x+1)=(x-1)/(x+1)