\[\cos{105{^\circ}} + \cos{100{^\circ}} + \cos{95{^\circ}} =\]
\[= 2\cos{100{^\circ}} \cdot \left( \cos{5{^\circ}} + \frac{1}{2} \right) =\]
\[= 2\cos{100{^\circ}}\left( \cos{5{^\circ}} + \cos{60{^\circ}} \right) =\]