1. (P & Q) V (R & S) 2. R -> L / ~P -> L | 3. ~P assumption for conditional proof | 4. ~P V ~Q 3. addition | 5. ~(P & Q) 4. deMorgan's law | 6. R & S 1,5, disjunctive syllogism | 7. R 6, simplification | 8. L 2,7, modus ponens 8. ~P -> L lines 3-8 for conditional proof. Edwin