Proof by Inference: Solving p-->s w/ Rules of Inference

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In summary, the conversation discusses a problem where the task is to prove or provide a counterexample for the statement: "If p, then q" and "(q and r), then s". The individual is struggling to use the law of syllogism to solve the problem, but is advised to start by assuming the precedent and proving the conclusion. With this approach, the individual is able to successfully provide a proof.
  • #1
ghostskwid
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Homework Statement


Asked to either prove or make a counter example:

p ----> q
(q and r) ----> s
r
-------------------------------
p------> s

Homework Equations





The Attempt at a Solution



I am having trouble making the step to the law of syllogism I know I need to solve this. Can anyone help me with stepping this our using the rules of inference. I would greatly appreciate this thanks.
 
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  • #2
Usually my first instinct when I need to prove an implication is to assume the precedent and prove the conclusion. I.e. in this case, try to prove

[itex]p \implies q[/itex]
[itex](q \wedge r) \implies s[/itex]
[itex]r[/itex]
[itex]p[/itex]
-------------------------------
[itex]s[/itex]

I think (hope) the next step is more obvious when put like this.
 
  • #3
Thanks that, got the proof based on that info. :D
 

1. What is proof by inference?

Proof by inference is a method used in logic and mathematics to prove the validity of a statement or argument based on a set of given premises. It involves using rules of inference, which are logical principles that allow us to make deductions and draw conclusions from these premises.

2. How does proof by inference work?

In proof by inference, we start with a set of given premises and use rules of inference to derive new statements or conclusions. These rules dictate how we can manipulate and combine the premises to reach valid conclusions. The end result is a proof that shows how the original statement or argument is true based on the given premises.

3. What are the common rules of inference used in proof by inference?

Some common rules of inference include modus ponens, modus tollens, hypothetical syllogism, disjunctive syllogism, and constructive dilemma. These rules allow us to make logical deductions based on the structure of the premises.

4. What is the difference between proof by inference and direct proof?

In direct proof, we start with the premises and use logical steps to directly reach the conclusion. In proof by inference, we use rules of inference to manipulate the premises and derive new conclusions that ultimately prove the original statement or argument.

5. Can proof by inference be used in any type of argument or statement?

Yes, proof by inference can be used in any type of valid argument or statement. It is a fundamental tool in logic and mathematics and is often used to prove the validity of mathematical theorems or logical arguments.

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