已知等差数列{an}的公差为2,若a1,a3,a4成等比数列,则a2=( ) A.-4 B.-6 C.-8 D.-10
问题描述:
已知等差数列{an}的公差为2,若a1,a3,a4成等比数列,则a2=( )
A. -4
B. -6
C. -8
D. -10
答
∵a4=a1+6,a3=a1+4,a1,a3,a4成等比数列,
∴a32=a1•a4,
即(a1+4)2=a1×(a1+6),
解得a1=-8,
∴a2=a1+2=-6.
故选B.