)Hướng dẫn trả lời
Ta có\(\eqalign{
& \overrightarrow {OM} = {1 \over 2}\overrightarrow {OA} = {1 \over 2}\overrightarrow {OA} + 0.\overrightarrow {OB} \,\,\,\,\, \Rightarrow \,m = {1 \over 2},\,n = 0. \cr
& \overrightarrow {MN} = \overrightarrow {ON} - \overrightarrow {OM} = {1 \over 2}\overrightarrow {OB} - {1 \over 2}\overrightarrow {OA} = \left( { - {1 \over 2}} \right)\overrightarrow {OA} + {1 \over 2}\overrightarrow {OB} \,\,\,\,\, \Rightarrow \,m = - {1 \over 2},\,n = {1 \over 2}. \cr
& \overrightarrow {AN} = \overrightarrow {ON} - \overrightarrow {OA} = {1 \over 2}\overrightarrow {OB} - \overrightarrow {OA} = \left( { - 1} \right)\overrightarrow {OA} + {1 \over 2}\overrightarrow {OB} \,\,\,\, \Rightarrow \,m = - 1,\,n = {1 \over 2}. \cr
& \overrightarrow {MB} = \overrightarrow {OB} - \overrightarrow {OM} = \overrightarrow {OB} - {1 \over 2}\overrightarrow {OA} = \left( { - {1 \over 2}} \right)\overrightarrow {OA} + \overrightarrow {OB} \,\,\,\, \Rightarrow \,m = - {1 \over 2},\,n = 1. \cr} \)
