等差数列an公差不等于0且a1,a3,a9成等比数列,求(a1+a3+a9)除(a2+a4+a10)

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  • 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