25 letters , it is not easy to do. I got this anwser in ten Mts.

來源: 2011-03-21 12:26:37 [舊帖] [給我悄悄話] 本文已被閱讀:

I think you can't solve the question about 25 letters. If a child can do it,(s)he would be genius.Please see my proof, it is not easy.We use symmetry and induction,it is not easy for most people.