\[\ \boxed{\mathbf{751\ (751)}\mathbf{.}\mathbf{\ }Еуроки\ - \ ДЗ\ без\ мороки}\]
\[\textbf{а)}\ 1 + ( - 2) + 3 + ( - 4) + 5 + ( - 6) = - 1 + 3 + ( - 4) + 5 + ( - 6) =\]
\[= 2 + ( - 4) + 5 + ( - 6) = - 2 + 5 + ( - 6) = 3 + ( - 6) = - 3.\]
\[1 + ( - 2) + 3 + ( - 4) + 5 + ( - 6) = (1 + 3 + 5) + \left( ( - 2) + ( - 4) + ( - 6) \right) =\]
\[= 9 + ( - 12) = - 3.\]
\[\textbf{б)}\ 20 + ( - 18) + 16 + ( - 14) + 12 + ( - 10) =\]
\[= 2 + 16 + ( - 14) + 12 + ( - 10) = 18 + ( - 14) + 12 + ( - 10) =\]
\[= 4 + 12 + ( - 10) = 16 + ( - 10) = 6.\]
\[20 + ( - 18) + 16 + ( - 14) + 12 + ( - 10) =\]
\[= (20 + 16 + 12) + \left( ( - 18) + ( - 14) + ( - 10) \right) = 48 + ( - 42) = 6.\]