The given expression can be written as,
$\frac{1}{2}\left[ {(1 - \cos {2^\circ }) + (1 - \cos {4^\circ }) + ................ + (1 - \cos {{360}^\circ })} \right]$
$ = \frac{1}{2}\left[ {(1 + 1 + ........ + 1) - (\cos {2^\circ } + \cos {4^\circ } + ........... + \cos {{360}^\circ })} \right]$
There are 180 terms in the series.
$ = \frac{1}{2}\left[ {180 - \frac{{\cos \left( {{2^\circ } + \frac{{180 - 1}}{2} \times {2^\circ }} \right)\sin \left( {\frac{{180 \times {2^\circ }}}{2}} \right)}}{{\sin \left( {\frac{{{2^\circ }}}{2}} \right)}}} \right]$
$ = \frac{1}{2}\left( {180 - \frac{{\cos {{181}^\circ }\sin {{180}^\circ }}}{{\sin {1^\circ }}}} \right) = 90$
