\[\boxed{\text{479\ (479).}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[a < 0;\ \ b < 0.\]
\[\textbf{а)}\ \sqrt{\text{ab}} = \sqrt{- a} \cdot \sqrt{- b}\]
\[\textbf{б)}\ \sqrt{\frac{a}{b}} = \frac{\sqrt{- a}}{\sqrt{- b}}\ \]