Question

已知等差数列{an}中,a2=3,a5=9,bn=2^an,则数列{bn}的前4项和S4=

Answer 1

A5-A2=3d=9-3=6

d=2

A1=A2-d=3-2=1

An=1+2(n-1)=2n-1

Bn=2^An=2^(2n-1)=2×2^(2n-2)=2×4^(n-1)

{Bn}是首项为2,公比为4的等比数列

S4=2×(1-4^4)/(1-4)=170