等差数列{an}的公差不为零,a4=7,a1,a2,a5成等比数列. 若数列{Tn}满足Tn=a2+a4+a8+…+,求Tn.

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问题 等差数列{an}的公差不为零,a4=7,a1,a2,a5成等比数列.
若数列{Tn}满足Tn=a2+a4+a8+…+,求Tn

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答案Tn=a2+a4+a8+…+a2n =(2×2-1)+(2×22-1)+(2×23-1)+…+(2×2n-1) =2×(2+22+23+…+2n)-n =[*]326 =2n+2+2-n-4

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