\[\boxed{\text{716\ (716).\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[1)\ c^{6} + c^{9} = c^{6}\left( 1 + c^{3} \right) =\]
\[= c^{6}(1 + c)(1 - c + c^{2})\]
\[2)\ m^{9} - n^{9} =\]
\[= \left( m^{3} - n^{3} \right)\left( m^{6} + m^{3}n^{3} + n^{6} \right) =\]
\[3)\ a^{8} - b^{4} =\]
\[= \left( a^{4} - b^{2} \right)\left( a^{4} + b^{2} \right) =\]
\[= (a^{2} - b)(a^{2} + b)\left( a^{4} + b^{2} \right)\]