宋小虎369
春芽
共回答了24个问题采纳率:95.8% 举报
1) bn都是正数 ,所以Tn》T1 = b1 =1/3
2) 裂项
bn=1/(n^2+2n)=1/(n(n+2))=[1/n - 1/(n+2)]/2
b1 = (1-1/3)/2
b2 =(1/2-1/4)/2
b3 =(1/3-1/5)/2
...
bn =[1/n - 1/(n+2)]/2
累加后
所以Tn = [1+1/2 - 1/(n+1) -1/(n+2)]/2 < (1+1/2) = 3/4
所以 1/3
1年前
2