Aplic formula:
[tex]\cos(\pi - x) = -\cos x[/tex]
Rezolvare:
[tex]\cos10^\circ +\cos20^\circ+\cos160^\circ+\cos170^\circ = \\ \\ = \cos10^\circ +\cos20^\circ+\cos(180^\circ-20^\circ)+\cos(180^\circ-10^\circ) = \\ \\ = \cos10^\circ+\cos20^\circ-\cos 20^\circ-\cos10^\circ = \\ \\ = 0[/tex]
