圖片參考：http://imgcld.yimg.com/8/n/HA00799676/o/701111200102113873403570.jpg
更新1:
To wy: I understand the way of doing that. But like in the first sight, when you see 1- z^-3, how do you know using the factor (1-z^-1) and (1+z^-1 + z^-2) Like in 1 - x^3 , how do you know it is (1 - x)( 1 + x + x^2) when you first see it?