设等差数列{an}的公差为d,
∵ a1,a3,a9成等比数列
∴ a3²=a1*a9
∴ (a1+2d)²=a1*(a1+8d)
∴ a1²+4a1d+4d²=a1²+8a1d
∴ 4d²=4a1d
∵ d≠0
∴ a1=d
∴ an=a1+(n-1)d=nd
∴ (a1+a3+a9)除(a2+a4+a10) 你确认是除,不是除以?
= (a2+a4+a10)/(a1+a3+a9)
=( 2d+4d+10d)/(d+3d+9d)
= 16d/(13d)
= 16/13
如果是除以
(a1+a3+a9)除以(a2+a4+a10) 你确认是除,不是除以?
= (a1+a3+a9)/(a2+a4+a10)
=( d+3d+9d)/(2d+4d+10d)
= 13d/(16d)
= 13/16