Question

已知等比数列{a_n}的前n项和为S_n,a₁=1,且满足S_n,S_n+2,S_n+1成等差,则a₃等于

A.                 B.               C.                   D.

Answer 1

C  ∵S_n,S_n+2,S_n+1成等差,∴S_n+2-S_n=S_n+1-S_n+2.

∴a_n+2+a_n+1=-a_n+2.∴=.又a₁=1,∴a₃=.