# How do you multiply -2sqrt10*sqrt8?

$- 2 \sqrt{10} \cdot \sqrt{8} = - 2 \sqrt{10 \cdot 8} = - 2 \sqrt{80}$
$= - 2 \sqrt{16 \cdot 5} = - 2 \sqrt{16} \sqrt{5} = - 2 \cdot 4 \sqrt{5} = - 8 \sqrt{5}$
...using $\sqrt{a} \cdot \sqrt{b} = \sqrt{a \cdot b}$