MATH220 December 2009
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Question 02 (a)

Write the negation of the following statement:
 $\forall {}x\in \mathbb {Z} ,\quad {\textrm {if}}\quad {}x>0\quad {\textrm {then}}\quad \exists {}y\in \mathbb {Q} \quad {\textrm {s.t.}}\quad {}x^{2}>y.$

Make sure you understand the problem fully: What is the question asking you to do? Are there specific conditions or constraints that you should take note of? How will you know if your answer is correct from your work only? Can you rephrase the question in your own words in a way that makes sense to you?

If you are stuck, check the hints below. Read the first one and consider it for a while. Does it give you a new idea on how to approach the problem? If so, try it! If after a while you are still stuck, go for the next hint.

Hint 1

Remember that bringing negations across for all statements changes them to exists statements and vice versa.

Hint 2

Remember that $A\Rightarrow B$ (if A then B) is logically equivalent to $\displaystyle \neg A{\text{ or }}B$

Hint 3

Remember that
 $\neg (A{\text{ or }}B)\equiv \neg A{\text{ and }}\neg B.$
(The left and right hand sides above are logically equivalent)

Checking a solution serves two purposes: helping you if, after having used all the hints, you still are stuck on the problem; or if you have solved the problem and would like to check your work.
 If you are stuck on a problem: Read the solution slowly and as soon as you feel you could finish the problem on your own, hide it and work on the problem. Come back later to the solution if you are stuck or if you want to check your work.
 If you want to check your work: Don't only focus on the answer, problems are mostly marked for the work you do, make sure you understand all the steps that were required to complete the problem and see if you made mistakes or forgot some aspects. Your goal is to check that your mental process was correct, not only the result.

Solution

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Following the hints, first rewrite the original using hint 3 as
 $\forall {}x\in \mathbb {Z} ,\quad \neg (x>0)\quad {\textrm {or}}\quad \exists {}y\in \mathbb {Q} \quad {\textrm {s.t.}}\quad {}x^{2}>y.$
Now, taking the negation:
 $\neg (\forall {}x\in \mathbb {Z} ,\quad \neg (x>0)\quad {\textrm {or}}\quad \exists {}y\in \mathbb {Q} \quad {\textrm {s.t.}}\quad {}x^{2}>y).$
Bring the negative in one level:
 $\exists {}x\in \mathbb {Z} ,\quad \neg (\neg (x>0)\quad {\textrm {or}}\quad \exists {}y\in \mathbb {Q} \quad {\textrm {s.t.}}\quad {}x^{2}>y)).$
Distribute across the bracket using the hints:
 $\exists {}x\in \mathbb {Z} ,\quad x>0\quad {\textrm {and}}\quad \neg (\exists {}y\in \mathbb {Q} \quad {\textrm {s.t.}}\quad {}x^{2}>y).$
Bring the negative inside:
 $\exists {}x\in \mathbb {Z} ,\quad x>0\quad {\textrm {and}}\quad \forall {}y\in \mathbb {Q} \quad {}x^{2}\leq y.$

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MER QGH flag, MER QGQ flag, MER QGS flag, MER RT flag, MER Tag Logic, Pages using DynamicPageList3 parser function, Pages using DynamicPageList3 parser tag

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