📘 logic and set theory

1. **Énoncé du problème :** - c. \forall x \in \mathbb{R}, \forall y \in \mathbb{R}, x + y > 0

1. Verify whether the statement $\sim(\sim q \to p) \to \sim q$ is a tautology. - Recall the implication equivalence: $a \to b \equiv \sim a \lor b$.