Question

已知｛an｝是等比例,且1是2分之a2和3分之a3的等差中项,6是2a2和3a3等比中项.

Answer 1

a2 /2 - 1= 1-a3 /3, 2 a2 *3 a3 =36,

a2 =2 a3=3

等比q=3/2 a1= 4/3 an=4/3(3/2)^(n-1)

2、 San=[a1(1-q^n)]/(1-q)=8/3(2/3^n-1)

2n是前n项和为N,N8/3(2/3^n-1)

B=San+N=8/3(2/3^n-1)+8/3(2/3^n-1)