an=6n-5(n是正整数)bn=3/(an*an+1)Tn是数列bn的前n项和,求使Tn

Answer 1

Tn =1/2[1 - 1/7 +1/7 -1/13 +1/13 -.-1/(6n+1)]
= 1/2 - 1/(12n+2）
后面是啥子,看清了
1/2 - 1/(12n+2） < m/20
==>m > 10 -10/(6n+1)
对所有n∈N+都成立
最小正整数m=10