1/9 = 0.111 ....
2/9 = 0.222 ....
3/9 = 0.333 ....
4/9 = 0.444 ....
5/9 = 0.555 ....
6/9 = 0.666 ....
7/9 = 0.777 ....
8/9 = 0.888 ....
9/9 = 0.999 .......... <------ ???
無窮等比級數 : 0.999...=0.9+0.09+0.009+0.0009+...
首項是0.9,公比為0.1的無窮等比級數
其和就是(首項)/(1-公比)=0.9/(1-0.1)=1
So 9/9 = 0.999 ... = 1 <------ ???