Prove the following theorem. (5 pts.) 28. - (3x)(FxGx) ((x) Fx-(3x) Gx) One of the following arguments is valid; the other isn't. Construct a proof for the valid argument. Construct a counterexample, either by the natural interpretation method or model universe method, for the invalid argument. (6 pts.) (a) (x) Fx V (x) Gx /:. (x)(Fx VGx) (b) (x)(Fx V Gx)/:. (x) Fx V (x) Gx Prove the following theorem. (5 pts.) 28. - (3x)(FxGx) ((x) Fx-(3x) Gx) One of the following arguments is valid; the other isn't. Construct a proof for the valid argument. Construct a counterexample, either by the natural interpretation method or model universe method, for the invalid argument. (6 pts.) (a) (x) Fx V (x) Gx /:. (x)(Fx VGx) (b) (x)(Fx V Gx)/:. (x) Fx V (x) Gx