a1,a2,a4成等比数列
(a2)^2=a1*a4
(a2)^2=(a2-d)(a2+2d)
(a2)^2=(a2)^2+a2d-2d^2
a2d=2d^2
a2=2d
(a1+a2+a4)／(a2+a4+a8)
=(a2-d+a2+a2+2d)/(a2+a2+2d+a2+6d)
=(3a2+d)/(3a2+8d)
=(3*2d+d)/(3*2d+8d)
=7d/(14d)
=1/2