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2009 homework set 4

# 2009 homework set 4 - PHIL 242 Homework Set 4 Due Monday...

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PHIL 242 Homework Set 4 Due Monday September 28 by 9 a.m. [All problems are worth 2 points each] 1. Problems 6.9, 6.12, 6.14, page 162 2. Problem 6.19, p. 166. 3. Problem 6.24, p.172. 4. Problem 6.40, p. 175 ________________________________ Strategies for Proofs 1. If you need to prove a sentence having the form of a conjunction, prove each conjunct separately, then use H Intro. 2. If you have a premise of the form of a disjunction and you need to prove a sentence D, try deriving D from a sequence of sub-proofs, each of which has one of the disjuncts as its assumption. Then use the rule of H Elim to get D as your main conclusion. 3. To prove a sentence of the form HD, begin a sub-proof with D as its assumption, derive a contradiction, then use H Intro to get HD as the first line after the end of the sub-proof. Alternatively, it is sometimes useful in deriving D to begin a sub-proof with HD as its assumption, derive H D by the above strategy, and then use H Elim to get D. 4. At any point in a proof or sub-proof, if you can derive a contradiction H you can use the rule
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