(2\operatorname{sen} a)(2 + 3\operatorname{sen} a) = 4\operatorname{sen} a + 6\operatorname{sen}^2 a,
(3\cos a)(1 - 2\cos a) = 3\cos a - 6\cos^2 a.
Passo 3
Subtraindo e agrupando termos dá
4\operatorname{sen} a + 6\operatorname{sen}^2 a - \bigl(3\cos a - 6\cos^2 a\bigr) = 4\operatorname{sen} a + 6\operatorname{sen}^2 a - 3\cos a + 6\cos^2 a.
Agora usamos a identidade trigonométrica \operatorname{sen}^2 a + \cos^2 a = 1, logo
6\operatorname{sen}^2 a + 6\cos^2 a = 6\cdot1 = 6.