[tex]I = \left\{x\in \mathbb{R}\big|\, 1 < x < 3\right\}\\ \\ a)\,\,\,I = (1, 3)[/tex]
[tex]b) \,\,\, \frac{15}{6} \in (1,3) \Rightarrow 1 < \frac{15}{6}< 3\big|\cdot 6 \Rightarrow 6 < 15 < 18 \,\,\,(A)[/tex]
[tex]c)\,\,\, \sqrt{1} < \sqrt{2} < \sqrt{9} \Rightarrow 1 < \boxed{\mathbf{\sqrt{2}}}<3[/tex]
