Originally Posted by

**epicurean** I'm going over old test results, and for whatever reason I just can't get this one into my head.

**Given the proposition**

$\displaystyle (p \rightarrow q)\rightarrow(p \vee r \rightarrow q \vee r) $

determine whether it is a tautology [it is, of course]**, and, if it is a tautology, write the valid argument determined by it.**

In other words, figure out what rule of inference can be derived from this argument. I believe it is *modus tollens*, but am not sure, and just can't see what method I would use to go about this. Any ideas?

Thanks to all the good people out there!