Chapter 7 - Section 7.7 - Complex Numbers - Exercise Set - Page 463: 19

$-5\sqrt 2$

$\sqrt -5 \times \sqrt -10=\sqrt (25\times-1\times-1\times2)=\sqrt 25\times\sqrt -1\times\sqrt -1\times\sqrt 2=5\times i\times i\times\sqrt 2=5\times i^{2}\times\sqrt 2=-5\sqrt 2$ We know that $\sqrt -1=i$ and that $i^{2}=-1$.