“For each of the following arguments, construct both a formal proof of validity and an indirect proof and compare their length”
1.
Formal Proof
- A ∨(B ∧ C)
- A→C …Therefore, C
- ¬A→(B ∧ C) (1,IMP)
- ¬C→¬A (2,TRANS)
- ¬C→(B ∧ C) (4,3,HS)
- C ∨ (B ∧ C) (5,IMP)
- (C ∨ B) ∧ (C ∧ C) (6,DIST)
- C ∧ C (7,SIMP)
- C (8,TAUT)
Indirect Proof
- A ∨(B ∧ C)
- A→C …Therefore, C
- ¬C (IP)
- ¬A (3,2,MT)
- B ∧ C (4,1,DS)
- C (5,SIMP)
- C ∧ ¬C (6,3,CONJ)
2.
Formal Proof
Indirect Proof
- (D ∨ E)→(F→G)
- (¬G ∨ H)→(D ∧ F) …Therefore, G
- ¬G (IP)
- ¬G ∨ H (3,ADD)
- D ∧ F (4,2,MP)
- D (5,SIMP)
- D ∨ E (6,ADD)
- F→G (8,1,MP)
- F (5,SIMP)
- G (9,8,MP)
- G ∧ ¬G (10,3,CONJ)
3. In back of book.
4.
Formal Proof
Indirect Proof
- (M ∨ N)→(O ∧ P)
- (O ∨ Q)→(¬R ∧ S)
- (R ∨ T)→(M ∧ U) …Therefore, ¬R
- R (IP)
- R ∨ T (4,ADD)
- M ∧ U (5,3,MP)
- M (6,SIMP)
- M ∨ N (7,ADD)
- O ∧ P (8,1,MP)
- O (9,SIMP)
- O ∨ Q (10,ADD)
- ¬R ∧ S (11,2,MP)
- ¬R (12,SIMP)
- ¬R ∧ R (13,4,CONJ)
5. In back of book.
∧ = And
∨ = Or
→ = If…then
¬ = Not (negation)
≡ = Logical Equivalence