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Who played first? (Posted on 2004-05-07) Difficulty: 3 of 5
Given this tic-tac-toe position, as played by two expert players, who went first and where? Also, which was the last "move"?
 X | O | O 
---+---+---
   |   | X
---+---+---
   | X | O
(An expert player is a player who would never play in such a way that would allow his opponent to win, and who would also try to get the possible best result.)

See The Solution Submitted by e.g.    
Rating: 3.7000 (10 votes)

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Solution Starting to work... | Comment 17 of 26 |
First, if it was X's turn, he could play middle left, and win -- since all matches should be draws, then it follows that it must be O's turn, and O played first.

If O had played at a corner, X would have had to answer in the center -- or otherwise O could have forced a win. So the first question is completely answered thus: "O started, playing top middle."

X couldn't have answered middle right, for O would won playing center. However, X playing top left or bottom middle do not allow for wins.

I found out more than a possible sequence, but they all end with X playing middle right, so that's the second answer.
  Posted by Federico Kereki on 2004-05-09 21:58:13

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