已知等差数列{an}的公差为3,若a2,a4,a8成等比数列,则a4等于( ) A.8 B.10 C.12 D.14
问题描述:
已知等差数列{an}的公差为3,若a2,a4,a8成等比数列,则a4等于( )
A. 8
B. 10
C. 12
D. 14
答
由a2,a4,a8成等比数列可得a42=a2a8
(a1+9)2=(a1+3)(a1+21)
整理可得,6a1=18,即a1=3
由等差数列的通项可得,a4=a1+d(3-1)=3+3×3=12
故选C