已知x+2,13-x,和17+x是某等比數列的首3項

(13-x)/(x+2) = (17+x)/(13-x)
(13-x)^2 = (17+x)(x+2)
x^2 - 26x + 169 = x^2 + 19x + 34
45x = 135
x = 3
5, 10, 20 ....
S(n) = (5)(2^n - 1)/(2-1) = (5)(2^n - 1)
S(10) = (5)(2^10 - 1) = 5115

2007-10-14 15:31:50 補充：
上面位做錯左, 等比 唔係等差

等差 = 13-x - (x+2) = 17+x-(13-x)
11-2x = 4+2x
7 = 4x
x = 7/4

首項 = 7/4 + 2 = 15/4, 等差 = 13-7/4 - 15/4 = 15/2
首10項之和 = 10/2 [2(7/4) + (10-1)(15/2)] = 5*(7/2+9*15/2) = 355