If it is from Ps Q, We can do like that

來源: 於 2012-01-02 11:12:38 [舊帖] [給我悄悄話] 本文已被閱讀：次

alfa=20d, beta=2d.

From second ,reflects 1 time, the angle increases 2d,so,n=36 the angle is 90d,that means light goes back the same road,the answer shoud be 72. 70,71,also OK,It is about what is first and last...

If it is strict Math, We have to think about the length,it is boring,...