1.1/n(n+1)=1/n-1/(n+1)
等式右边=1/n-1/(n+1)
通分=(n+1-n)/n(n+1)=等式左边
所以成立
2.原式=1/(x-3)-1/(x-2)-1/(x-3)+1/(x-1)+1/(x-2)-1/(x-1)
=0