x²+2x+1=(x+1)²
x²-1=(x+1)(x-1)
所以最简公分母是(x+1)²(x-1)
所以
(x-1)/(x²+2x+1)=(x-1)²/(x+1)²(x-1)
2/(x²-1)=2(x+1)/(x+1)²(x-1)