等差等比数列题有4个数前三项成等比数列,和为19后三项成等差数列求这4个数

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  • 你确认题目没有写错吧等差数列                  等比数列                 前k项和a(1) = a1                                 b(1) = b1a(2) = a1 + d                           b(2) = b1*da(3) = a1 + 2d                         b(3) = b1*d²......                                          ......a(n) = a1 + (n-1)d                   b(n) = b1*d^(n-1)根据题意:a1=b1                           ①a(4) = b(4)       ===> a1 + 3d = b1*d³             ②a(10)=b(10)     ===> a1 + 9d = b1*d^9       ③联立解得:a1 = 2^(1/3)                 d = - 2^(1/3)        或者  a1 = 0                         d = 0    --------   不知道现在的教材是不是该舍去?你来定吧b(16) = a1*d^15 =  -2^(16/3)假设有某一项 a(k+1) = b(16)即 2^(1/3) (1 + k) =  - 2^(16/3)则  k +1 = - 2^5 k = - 33  不是自然数,所以b16  不是an 中的项