1. (~B v A) -> (C & D) 2. A -> ~C 3. ~A -> ~B /:. ~(B <-> C) --------------------------- 4. ~(~B v A) v (C & D) 1 Material Implication 5. (~~B & ~A) v (C & D) 4 De Morgan's Law 6. (B & ~A) v (C & D) 5 Double Negation 7. [(B & ~A) v C] & [(B & ~A) v D] 6 Distribution 8. [C v (B & ~A)] & [D v (B & ~A)] 7 Commutation 9. C v (B & ~A) 8 Simplification 10. (C v B) & (C v ~A) 9 Distribution 11. C v B 10 Simplification 12. B v C 11 Commutation 13. ~~B v C 12 Double Negation 14. ~B -> C 13 Material Implication 15. B -> A 3 Transposition 16. B -> ~C 15,2 Hypothetical Syllogism 17. (B -> ~C) & (~B -> C) 16,14 Conjunction 18. (~B v ~C) & (~~B v C) 17 Material Implication 19. (~B v ~C) & (~~B v ~~C) 18 Double Negation 20. ~(B & C) & ~(~B & ~C) 19 De Morgan's Law 21. ~[(B & C) v (~B & ~C)] 20 De Morgan's Law 22. ~(B <-> C) 21 Material Equivalence