Prove that the following arguments are invalid.
1. (∃x) (Ax * ~Bx)
2. (∃x) (Ax * ~Cx)
3. (∃x) ( ~Bx * Dx)
/ (∃x) [Ax *(~Bx * Dx)]
Answer :
1. (∃x) (Ax * ~Bx) Premise
2. (∃x) (Ax * ~Cx) Premise
3. (∃x) ( ~Bx * Dx) Premise
4. Aa * ~Ba (1) , Existential Instantiation
5. Aa (4) , Simplification
6. ~Ba (4) , Simplification
7. Aa * ~Ca (2) , Existential Instantiation
8. Aa (7) , Simplification
9. ~Ca (7) , Simplification
10. ~Ba * Da (3) ,Existential Instantiation
11. Aa *(~Ba * Da) (10), Simplification
12. (∃x) [Ax *(~Bx * Dx)] (11), Existential Generalization
Thus , the conclusion (∃x) [Ax *(~Bx * Dx)] is derived from the given premises and hence the argument is valid
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