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Unformatted text preview: CISC 404/604 Homework 1 Solutions 1a. ( B → ( C ∨ D )) is valid and C is unsatisfiable. To prove B → D is valid: Suppose bwoc B → D is not valid. ∴ ∃ a truthvalue assignment, say ν 1 , such that ν 1 ( B ) = T (1) & ν 1 ( D ) = F [by defns of validity and → ]. (2) Since ( B → ( C ∨ D )) is valid, ∴ ν 1 ( B → ( C ∨ D )) = T [by defn of validity]. Since ν 1 ( B ) = T [by (1)], ∴ ν 1 ( C ∨ D ) = T [by defn of → ]. Since C is unsatisfiable, ∴ ν 1 ( C ) = F [by defn of satisfiable]. ∴ ν 1 ( D ) = T [by defn of ∨ ], which is a contradiction [cf. (2)]. ∴ ( B → D ) is valid [proof by contradiction]. 1b. Γ 1  = B , ∴ ∀ ν , if ν satisfies Γ 1 , then ν ( B ) = T [by defn of  =]. (1) Γ 2  = C , ∴ ∀ ν , if ν satisfies Γ 2 , then ν ( C ) = T [by defn of  =]. (1) ∀ ν , if ν satisfies Γ 1 ∪ Γ 2 , then ν also satisfies Γ 1 and Γ 2 [by defns of satisfies and ∪ ]. ∴ ∀ ν , if ν satisfies Γ 1 ∪ Γ 2 , then ν ( B ) = T and ν ( C ) = T [by (1) & (2)]. ∴ ∀ ν , if ν satisfies Γ 1 ∪ Γ 2 , then ν ( B ∧ C ) = T [by defn of ∧ ]....
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This document was uploaded on 12/07/2011.
 Spring '09

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