\[\boxed{\mathbf{505\ (505).\ }Еуроки\ - \ ДЗ\ без\ мороки}\]
\[1)\ \sqrt{\text{ab}} = \sqrt{a} \cdot \sqrt{b}\ \ \ при\ \ a \geq 0,\]
\[\ b \geq 0;\]
\[2)\ \sqrt{\text{ab}} = \sqrt{- a} \cdot \sqrt{- b}\ \ \ при\ \ a \leq 0,\]
\[b \leq 0;\]
\[3)\ \sqrt{- ab} = \sqrt{a} \cdot \sqrt{- b}\ \ при\ \ \ a \geq 0,\]
\[b \leq 0.\]
\[\boxed{\mathbf{50}\mathbf{5}\text{.}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[1)\ \frac{1}{6} < 0,2 = \frac{1}{5}\text{\ \ }\]
\[2)\ \frac{7}{9} = 0,(7) > 0,77\ \ \]
\[3) - 1,(645) < - 1,(643)\text{.\ }\]