# hw3-sol - CS 173 Discrete Structures Spring 2010 Homework 3...

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CS 173: Discrete Structures, Spring 2010 Homework 3 Solution This homework contains 3 problems worth a total of 40 points. When a problem speciFes a particular proof technique, you must use that technique in your solution, even if it’s not the only reasonable approach to the mathematical problem. This is because the main point of these problems is to learn how to use the various di±erent proof techniques. 1. [8 points] Thinking about negations Using precise mathematical notation, give the negations of the following statements. (a) ! x R , P ( x ) [Solution] This is equivalent to saying: ( x R , P ( x )) ( y R , P ( y ) ( y = x )) The negation of which is: ¬ (( x R , P ( x )) ( y R , P ( y ) ( y = x ))) ¬ ( x R , P ( x )) ∨ ¬ ( y R , P ( y ) ( y = x )) ( x R , ¬ P ( x )) ( y R , ¬ ( ¬ P ( y ) ( y = x ))) ( x R , ¬ P ( x )) ( y R , ( P ( y ) ( y n = x ))) Or, in English: P is either not true for any real number, or true for more than one real number. (b) p Z and m Z and n Z and p is not the greatest common divisor of m and n [Solution] p / Z m / Z n / Z p = GCD ( m, n ) You can use any combination of mathematical shorthand or mathematical English (e.g., for the logical operations) but be clear about the order in which operations are applied using parentheses, indentation or line breaks. 2. [22 points] A triple ( a, b, c ) of positive integers is Pythagorean if a 2 + b 2 = c 2 .

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## This note was uploaded on 11/09/2011 for the course CS 173 taught by Professor Erickson during the Spring '08 term at University of Illinois, Urbana Champaign.

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hw3-sol - CS 173 Discrete Structures Spring 2010 Homework 3...

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