# Basic Propositional Calc

1. Nov 12, 2007

### Steverino777

This is mostly some basic stuff, but I just want to make sure I am doing these right. I have a hard time understanding what the questions are saying sometimes.

1. The problem statement, all variables and given/known data
1)Explain how we may conclude that if f is a decreasing function, then f(5) < f(3)
Make reference to a logical principle. [Note- By defintion, a function g is decreasing i.f.f the conditional "if x<y, then g(x)>g(y)" is true for any real numbers x and y]

2)Suppose A and B are sets and w is an object, for which it is known that A$$\subseteq$$B and w$$\in$$A. Explain how we may conclude w$$\in$$(A$$\cap$$B) Make reference to a logical principle.

3)Explain why it is true that if a real number x satisfies |x|>a, but it is not the case that x > a, then x < -a must hold. Make reference to a logical principle.

2. Relevant equations
(p$$\wedge$$q) -> p Law of Simplification
p -> (p$$\vee$$q) Law of Addition
[p$$\wedge$$(p ->q)] -> q Modus Ponens
[(p$$\vee$$q)$$\wedge$$ ~q] -> p Modus Tollendo Ponens
[(p -> q)$$\wedge$$~q] -> ~p Modus Tollens
(p -> r) -> [(p$$\wedge$$q) -> r]
[~p -> (q$$\wedge$$~q] -> ~p Law of Contradiction

3. The attempt at a solution
1) The definition of a decreasing function states, "if x<y, then g(x)>g(y)" is true for any real numbers x and y. By hypothsis we know f is a decreasing function, so that the preticular case of the definition, "if 5 < 3, then g(5) > g(3)" is known to be true. --That's as far as I can get using an example form the book as a guideline. I'm not sure where to go from here and which logical principle applies to this argument.--

2) --I know how to explain it but I don't know which logical principle to use.--
A$$\subseteq$$ B includes all the elements in A that are also elements of B. Since it is given w$$\in$$ A, by this definiton w is also an element of B. A$$\cap$$B includes only the elements that A and B share in common. Since w is both an element of A and B, it is one of the elements that A and B share in common, making it an element of A$$\cap$$B.

3)If a real number x satisfies |x|>a, that means x < -a or x > a. It is also given that it is not the case that x > a. Therefore by Modus Tollendo Ponens we can conclude that x < -a.

If anyone could help me with finding which logical principles apply to one and two, that'd help a lot.

Last edited: Nov 12, 2007
2. Nov 12, 2007

### Steverino777

Anybody out there who could help?

3. Nov 12, 2007

### TalonStriker

I could help you with number 2, you could make use of the "intersection of a subset rule." But you've got the basic idea nailed.

Edit: I think that you could use Universal Modus Ponens for #1.

Last edited: Nov 12, 2007