cos(70°)+cos(110°)
2cos(90°)cos(-20°)
2×0cos(-20°)
0
[tex]\cos(70^{\circ})+\cos(110^{\circ}) = -\cos(180^\circ-70^{\circ}}) +\cos(110^{\circ}) = \\ \\ = -\cos(110^{\circ}) +\cos(110^{\circ})= 0\\ \\ \\\boxed{\cos(\pi - x) = -\cos x}[/tex]
