求等比級數1-1/2+1/4-1/8+...............無限項之和

1-1/2+1/4-1/8+...............
首項a = 1
公比R = -1/2
用a/(1-R)
1-1/2+1/4-1/8+...............
= 1/[1-(-1/2)]
= 1/[1+1/2]
= 2/3.

common rate=(-1/2)/1= -1/2
無限項之和=a/(1-r) (a=第一項,r=common rate)
無限項之和=1/(1+1/2)=1/(3/2)=2/3
so
無限項之和=2/3

笫2頂=1/4-1/8=1/8
笫1頂=1-1/2=1/2 公比=1/8 / 1/2=1/4
s(無限)= 笫一頂/ 1-公比
= 1/2 / 1-1/4
= 1/2 / 3/4
=2/3