Question

[数列问题]{an}是等差数列,公差D>0,Sn是{an}的前N项和,已知a1a4=22,S4=26

Answer 1

1 因为S4=4*(a1+a4)/2=2(a1+a4)=26

a1+a4=13 (1)

a1a4=22 (2)

由(1)得

a4=13-a1 带入(2)

a1(13-a1)=22

a1=11或a1=2

a1=11时,a4=2,d