等差数列{an}中公比为3,若a1,a3,a4成等比数列,则a2=?
问题描述：
等差数列{an}中公比为3,若a1,a3,a4成等比数列,则a2=?
答
公差为3
则a3=a1+2*3=a1+6
a4=a1+3*3=a1+9
a1,a3,a4成等比数列
则(a3)^2=a1*a4
(a1+6)^2=a1*(a1+9)
a1^2+12a1+36=a1^2+9a1
a1=-12
所以a2=a1+3=-12+3=-9