標題:

求n和首n項之和

發問:

1458,486,162,54...2

最佳解答:

1458, 486, 162, 54, … , 2 This is a geometric sequence First term, a = 1458 Common ratio, r = 486 / 1458 = 1 / 3 So, the nth term, Tn = 2 1458(1 / 3)n-1 = 2 (2 X 36) / 3n-1 = 2 2 / 3n-7 = 2 1 = 3n-7 30 = 3n-7 n = 7 Sn = S7 = 1458[1 – (1/3)7] / (1 – 1/3) = 2186

其他解答:

a=1458 r=1/3 ar^(n-1)=2 (1/3)^(n-1)= 2/1458 n-1=6 n=7 首n項之和 a (1 - r^n)/(1 - r)=1458(1- (1/3)^7)/(1-(1/3))=2186

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