點計~? 數例

等比列
-1/2+1/6-1/18+...
首項-1/2
公比-1/3
-1/2+1/6-1/18+...
=(-1/2)/[1-(-1/3)]
=(-1/2)*(3/4)
=-3/8

2006-12-30 17:51:42 補充：
請問咩叫首項之和 好似冇這個名詞﹐首n項之和就有即由第一項加到第n項

呢個係一個geometric series
-1/2, +1/6, -1/18,...

Common ratio (r) = - 1 / 3

Sum to infinity (無限項之和)
= a / (1 - r)
= (- 1 / 2) / (1 - (- 1 / 3))
= - 3 / 8

另外, 係咪想問 "首n項之和"??
例如, 首5項之和 = T(1) + T(2) + T(3) + T(4) + T(5)