I'm so glad that find this useful site. anyway, I ran into some challenging ways to find a formula is valid. Here is two example in my note that called valid. I ran into such a problem with making interpretation, anyone could help me what is the better way to showing these ones is valid !?1
(1) $ \exists y \forall x p(x,y) \to \forall x \exists y p(x,y) $
(2) $ \forall x \forall y p(x,y) \to \exists x \exists y p(x,y) $