Is AB // CD?give the reason

我曾在意見欄解釋過，建議貼到另一網站，但被指責為違規。

2014-07-29 14:52:20 補充：
Your figure cannot be seen.

I include below the three conditions that you can show a pair of straight lines to be parallel.


圖片參考：https://s.yimg.com/rk/HA00430218/o/800290803.png

死 link，無野睇！唔怪得無人能答啦！