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Proof
by Contradiction:
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Given
KB, want to show KB Ö f (KB entails f)
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Let
KB’ = KB È {Øf}
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If we
can prove that KB’ Ö false (inconsistent,
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contradiction),
then we’ve proven that KB Ö f
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If
KB’ | false and our inference rules are
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sound
then, KB’ Ö false
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