13. (9) Solve the inequality $\sqrt{x^{2}-1}+\sqrt{x^{2}-4 x+3}+\sqrt{2 x+3-x^{2}} \geqslant 2$.
Answer: $x \in\{-1,1,3\}$.

Solution: Let's find the domain of the given inequality. It consists of 3 points: $\{-1,1,3\}$. By direct substitution, we can verify that all of them are valid.