8.在等差数列{an}中,若a1=2,a2+a3=13,则a4+a5+a6=多少?9.已知等差数列{an}的前n项和为Sn,若a2=1,S5=10,则S7=多少?10.已知{an}为等差数列,a3+a4=1,则其前6项之和为多少?12.已知数列{an}满足a1=1,且an=n(an+1-an)(n∈N)

Question

8.在等差数列{an}中,若a1=2,a2+a3=13,则a4+a5+a6=多少?
9.已知等差数列{an}的前n项和为Sn,若a2=1,S5=10,则S7=多少?
10.已知{an}为等差数列,a3+a4=1,则其前6项之和为多少?
12.已知数列{an}满足a1=1,且an=n(an+1-an)(n∈N),则an=多少?

Answer 1

8
a1+a2+a3=3a2=15
a2=5=a1+d
d=3
a4+a5+a6=3a1+12d=6+36=42

9
a2=a1+d=1
s5=5a1+10d=10
a1=0
d=1
s7=7a1+21d=21

10
a1+a6=a3+a4=1
所以S6=6(a1+a6)/2=3*1=3

12
an=n*a(n+1)-nan
(n+1)an=n*an+1
an+1/an=(n+1)/n
所以
an/an-1=n/(n-1)
an=an/an-1*(an-1/an-2)...(a2/a1)=an/a1=n