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$