Stii ca sin π/4 = -sin 3π/4 si cos 2π/3 = - cos π/3 . Deci rezultatul va fi 0. Sau metoda mai explicita :
In general, daca nu stii cos 120°, sin 135°, calculezi cu formulele sin(a+b), sin(a-b), cos(a-b), cos(a+b) ,,, depinde de caz.
cos 120° = cos 2π/3 = cos(π-π/3) = cos π · cos π/3 + sin π · sin π/3 = -1/2 + 0 = -1/2 .
sin 135° = sin 3π/4 = sin(π - π/4) = sin π · cos π/4 - cos π · sin π/4 = 0 - (-1 ·√2/2) = √2 / 2.
Deci
(cos120°+cos60°)(sin135°-sin45°) = (-1/2 + 1/2) (√2 / 2 - √2/2 ) = 0.
