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I'm having a hard time reducing expressions involving "implies" operators. I did some reading about the actual meaning of the "implies" operator and browse for other Q&A on this website; however, I don't know how to interpret expressions. For example, I have:

$(C1) A \implies B$ $(C2) C \implies B$

And I as a definition given in the context I know that: $ C \implies A$

Now, I'm being asked to prove or disprove: $ a) C1 \implies C2$ and $ b) C2 \implies C1$

Here is where my problem/question, for a) is it valid to do the following?

$ (A \implies B) \implies (C \implies B)$

and then (somehow):

$ A \implies C$

Based on my definition given a above, I'm thinking I could "disprove" a) and "prove" b) but I'm not sure if logic has any foundation.

Any help would be appreciated

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    I think I'm allowed to; however, I don't see a straight forward way of doing for my particular scenario. I'll keep it in mind, thanks!2012-04-09

1 Answers 1

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You know that $C\implies A$. If you also knew $C1$, you’d have $C\implies A\quad\text{ and }\quad A\implies B\;,$ from which you may certainly infer $C\implies B$, which is $C2$. Thus, given that $C\implies A$, you must have that $C1\implies C2$. We’ve just proved (a).

As for (b), does knowing that $C\implies A$ and $C\implies B$ guarantee that $A\implies B$? What if $A,B$, and $C$ are:

$\qquad C:\quad n$ is a multiple of $6$,

$\qquad A:\quad n$ is a multiple of $2$, and

$\qquad B:\quad n$ is a multiple of $3$?

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    Thank you guys, I guess this means $$(A \implies B) \implies (C \implies B)$$ cannot be reduced to: $A \implies C $2012-04-09