Consider the theorem and proof below: theorem: -(pa-q) = -pvq proof: consider the truth table for the two statements. since the truth table is correct and the truth values match, the two statements are equivalent. p q -p -q pa-q (pa-q) -pvq t t f f f t fttfft t | т т т т т т қ т is the proof valid, and which type of proof is used? proof by contradiction is used, and the proof is valid. proof by cases is used, but the proof is not valid. proof by cases is used, and the proof is valid. proof by contraposition is used, but the proof is not valid.