Question

求20题解题详细的过程和答案，谢谢！

解题过程和答案最好写在一张纸上，写清楚点，拍下来

Answer 1

a3=9 a2*a5=243由于是等比数列(*是乘号)
所以a3＝a1*q² a2＝a1*q
a5等于a1*q的四次方
a2*a5=(a1)²乘以q的五次方=243 (1)
a3＝a1*q² =9 (2)
(1)(2)联立得 a1=1 q=3
an=1×3的(n-1)次方=3的(n-1)次方

Answer 2

(1)
an = a1.q^(n-1)
a3=9
a1.q^2=9 (1)
a2.a5=243
(a1)^2.q^5 =243 (2)
(2)/(1)^2
q= 243/81
= 3
from (1)
a1.q^2=9
a1=1
ie
an = 3^(n-1)
(2)
bn=log<3>an
=n-1
Sn
=b1+b2+...+bn
=n(n-1)/2