Question 610046: my question is that i got the same conclusion as my instructor but with different methods and i was wondering if that is still ok. the instructions are to add just 2 statements to the premises to produce a formal proof of validity. ill provide my answers and his, thanks a lot.
1. (S.T) v (U.V)
2. ~S v ~T
Therefore U.V
My answer
3. (U.V) v (S.t) 1, Com.
4. U.V 3, Taut.
his answer
3. ~(S.T) 2, Dem.
4. U.V 1,3 D.S.
Answer by jim_thompson5910(35256)   (Show Source):
You can put this solution on YOUR website! Your line 4 is incorrect. A tautology is of the form p v p or p & p and they both reduce to p.
But your line 3 doesn't fit that form. So you can't use a tautology.
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