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cs245-asn4-soln

# cs245-asn4-soln - CS 245 Winter 2009 Assignment 4 Due Thu 5...

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CS 245 Winter 2009 Assignment 4 Due: Thu 5 Mar 2009 10am in the CS245 Drop Boxes 40 marks SOLUTION SET There may be multiple correct answers to some of these questions. 1. (8 marks) Determine whether the following argument is valid or invalid. If it is valid, prove its validity using natural deduction. If it is invalid, provide a counterexample and demonstrate that the argument is invalid. Do not use any rules from transformational proof. x f ( x ) = g ( x,a ) , x P ( x ) Q ( f ( x )) | = ND x • ¬ P ( f ( x )) Q ( g ( f ( x ) ,x )) ( a is a constant) Soln: 1 x f ( x ) = g ( x,a ) premise 2 x P ( x ) Q ( f ( x )) premise 3 P ( f ( a )) ∨ ¬ P ( f ( a )) LEM 4 P ( f ( a )) assumption 5 P ( f ( a )) Q ( f ( f ( a ))) E 2 6 Q ( f ( f ( a ))) E 4 , 5 7 f ( f ( a )) = g ( f ( a ) ,a ) E 1 8 Q ( g ( f ( a ) ,a )) = E 6 , 7 9 ¬ P ( f ( a )) Q ( g ( f ( a ) ,a )) I 8 bracketleftbigg 10 ¬ P ( f ( a )) assumption 11 ¬ P ( f ( a )) Q ( g ( f ( a ) ,a )) I 10 12 ¬ P ( f ( a )) Q ( g ( f ( a ) ,a )) cases 3 , 4 9 , 10 11 13 x • ¬ P ( f ( x )) Q ( g ( f ( x ) ,x )) I 12 1

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2. (24 marks) Construct a theory about a tic-tac-toe-like game on a J × J grid, where J is a constant. The grid is marked with X ’s by one player and O ’s by the other player. Use the following functions and predicates as well as arithmetic predicates such as : wins ( x ) – means the player with mark x wins horiz row ( x ) – means the player with mark x has a horizontal line vert row ( x ) – means the player with mark x has a vertical line loc ( r,c ) – returns the mark at row r and column c in the grid A player wins if they have a horizontal or vertical line of their marks.
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