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

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,...

所有跟帖： 

• 回复：If it is from Ps Q, We can do like that -cma- ♂ (7782 bytes) () 01/02/2012 postreply 13:48:33

• dear cma,81is not good. -jinjing- ♀ (266 bytes) () 01/02/2012 postreply 16:54:15