Question

calculați cos70°+cos110°​

Answer 1

cos(70°)+cos(110°)

2cos(90°)cos(-20°)

2×0cos(-20°)

0

Answer 2

$\cos(70^{\circ})+\cos(110^{\circ}) = -\cos(180^\circ-70^{\circ}}) +\cos(110^{\circ}) = \\ \\ = -\cos(110^{\circ}) +\cos(110^{\circ})= 0\\ \\ \\\boxed{\cos(\pi - x) = -\cos x}$