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排列組合,定義“好字”為由三個字母A、B、C所組成的序列

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Define a good word as a sequence of letters that consists only of the letters A,B and C _  some of these letters may not appear in the sequence _ and in which A is never immediately followed by B, B is never immediately followed by C, and C is never immediately followed by A. How many seven-letter good words are there?
(2003AIME,http://www.artofproblemsolving.c ... 82&cid=45&year=2003)
這裡可以下載2003AIME中文版題目
http://forum.nta.org.tw/examservice/showthread.php?t=52834
http://forum.nta.org.tw/examserv ... =229485&postcount=5

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