等差数列的公差为3,若a2,a4,a8成等比数列,求a4

问题描述:

等差数列的公差为3,若a2,a4,a8成等比数列,求a4

a2=a1+3,
a4=a1+9,
a8=a1+21
a4/a2=a8/a4
(a1+9)/(a1+3)=(a1+21)/(a1+9)
a1=3
a4=3+9=12