幫忙補上出處
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，
https://artofproblemsolving.com/ ... _Problems/Problem_3)
這裡可以下載2003AIME中文版題目
連結已失效h ttp://forum.nta.org.tw/examservice/showthread.php?t=52834
連結已失效h ttp://forum.nta.org.tw/examservice/showpost.php?p=229485&postcount=5
