an=[(n+1)-n]/n(n+1)=(n+1)/n(n+1)-n/n(n+1)=1/n-1/(n+1)所以Sn=1-1/2+1/2-1/3+……+1/n-1/(n+1)=1-1/(n+1)=n/(n+1)=10/11n=10
