1/(n^2+n)=1/n(n+1)=1/n - 1/(n+1)1/2+1/6+1/12+1/20+...+1/n的平方+n=1/1-1/2+1/2-1/3+1/3-1/4+...+1/n-1/(n+1)=1 - 1/(n+1)=10/11n=10