Question

设数列{an}是公差为d（d＞0）的等差数列,Sn为{an}的前n项和,已知S4=24,a2乘a3=35,（1）求数列{

Answer 1

(1)S4=2(a2+a3),a2+a3=12

a2a3=35

t^2-12t+35=0

a2=5,a3=7

an=2n+1

(2)1/an(an+1)=(1/2)[1/(2n+1)-1/(2n+3)]

Tn=1/2(1/3-1/5+1/5-1/7+...-1/(2n+1)+1/(2n+1)-1/(2n+3))

=n/[3(2n+3)]