Question

1. Вычислить sina;tga;cos2a если:
а) соsa= -4/5, П/а <а <П

Answer 1

cos 2a = 2 (cos a)^2 - 1 = 2*(-4/5)^2 - 1 = 2*16/25 - 1 = 32/25 - 1 = 7/25

(sin a)^2 + (cos a)^2 = 1

sin a = $\sqrt[2]{1 - (-4/5)^2}$ = 3/5

tg a = sin a / cos a = 3/5 : (-4/5) = 3/5 * (-5/4) = -3/4