a)
\(\eqalign{
& 1 + \left( { - 3} \right) + 5 + \left( { - 7} \right) + 9 + \left( { - 11} \right) \cr
& = \left( {1 + 9} \right) + \left[ {\left( { - 3} \right) + \left( { - 7} \right)} \right] + 5 + \left( { - 11} \right) \cr
& = 10 + \left[ { - \left( {3 + 7} \right)} \right] + 5 + \left( { - 11} \right) \cr
& = 10 + \left( { - 10} \right) + 5 + \left( { - 11} \right) \cr
& = \left[ {10 + \left( { - 10} \right)} \right] + 5 + \left( { - 11} \right) \cr
& = 5 + \left( { - 11} \right) = - \left( {11 - 5} \right) = - 6 \cr} \)
b)
\(\eqalign{
& \left( { - 2} \right) + 4 + \left( { - 6} \right) + 8 + \left( { - 10} \right) + 12 \cr
& = \left[ {\left( { - 2} \right) + \left( { - 6} \right) + \left( { - 10} \right)} \right] + (4 + 8 + 12) \cr
& = \left[ { - \left( {2 + 6 + 10} \right)} \right] + 24 \cr
& = - 18 + 24 = 24 - 18 = 6 \cr} \)
