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標題:

Arithmetic and Geometric Sequence (1)

發問:

In an arithmetic sequence, T(4) + T(8) + T(12) + T(16) = 224. Find the sum of the first 19 terms of this sequence.

最佳解答:

T(4) + T(8) + T(12) + T(16) = 224 let T(n)=a+(n-1)d then T(4) + T(8) + T(12) + T(16) = 224 =>a+(3)d+a+7d+a+11d+a+15d=224 =>4a+36d=224 =>a+9d=56--(1) S(19)=19(2a+18d)/2=19*(a+9d)=19*56=1064//

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