Official solution
Given \[\,\,\,\,\left| z \right| = 1,\,\,\arg \,z = \theta \]
As we know, \[\,\,\,\,\overrightarrow z = {1 \over z}\]
\[\therefore\] \[\,\,\,\,\arg \left( {{{1 + z} \over {1 + \overrightarrow z }}} \right) = \arg \left( {{{1 + z} \over {1 + {1 \over z}}}} \right)\]
\[ = \arg \left( z \right) = \theta .\]