x/(x²+2x+1),(x-1)/(x²+x),1/(x²-1)
式1：x/(x+1)²,式2：(x-1)/[x(x+1)],式3：1/[(x+1)(x-1)]
通分结果：
[x²(x-1)]/[x(x-1)(x+1)(x+1)]；[(x-1)(x-1)(x+1)]/[x(x-1)(x+1)(x+1)]；[x(x+1)]/[x(x-1)(x+1)(x+1)]