3.
[tex]\sqrt{80} = \sqrt{16\cdot5} = \sqrt{16}\cdot\sqrt{5} = 4\cdot\sqrt{5} = \underline{4\sqrt{5}}[/tex]
4.
[tex]A. \ 2\sqrt{3}\times \sqrt{6} = 2\sqrt{3\times6} = 2\sqrt{18} = 2\sqrt{9\times2} = 2\cdot\sqrt{9}\cdot\sqrt{2}=2\cdot3\cdot\sqrt{2} = \underline{6\sqrt{2}}\\\\\\B. \ \frac{9\sqrt{10}}{3\sqrt{5}} = 3\sqrt{\frac{10}{5}} = \underline{3\sqrt{2}}[/tex]
