Definitions

• valid = tautology = always true

• satisfiable = sometimes true

• unsatisfiable = never true

1) smoke ? smoke

2) smoke ? fire

3) (smoke ? fire) ? (?smoke ? ?fire)

4) smoke ? fire ? ?fire

?smoke ? smoke valid

? smoke ? fire satisfiable

? (? smoke ? fire) ? (?smoke ? ?fire)

valid

(smoke ? ?fire) ? ? smoke ? ?fire valid

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