The following exercises each contains an argument; some are valid, some not. If the argument is valid, give an informal proof. If it is not valid, use Tarskis World to construct a counterexample.

12.9 tx (Cube(x) Large(x)) v (Tet(x) Small(x))] V. Tet(x) BackOf(x, c)] yx Small(x) BackOf(x, c)] 12.9 tx (Cube(x) Large(x)) v (Tet(x) Small(x))] V. Tet(x) BackOf(x, c)] yx Small(x) BackOf(x, c)]