Challenging Question # 3

Arrange the numbers 1 through 9 on a tic-tac-toe board, so that every row and every column and every diagonal adds up to the same number.  Show that there is only one way (ignoring rotations and reflections) to do this. 


The answer will be posted on 9/15/2011 Thursday 


Any comments:5 points 
Right answer: 15 points