bn=2/(n^2+n)=2[1/n-1/(n+1)]
b1+b2+.+bn=2(1-1/2+1/2-1/3+...1/n-1/(n+1))=2(1-1/(1+n))=2n/(n+1)
因为n/(n+1)大于0小于1所以2n/(n+1)小于2