Помогите 1 и 2 задание

1) sin a = -5/13 3π/2 < a  < 2π  ⇒ cos a > 0 sin² a + cos² a = 1 2)  cos 1290° = cos (3*360 + 210) = cos 210° = cos (180 + 30) = -cos 30° = -0,5√3 cos 15π/4 = cos [(16π - π) /4] = cos(4π - π/4) = cos (2* 2π - π/4) = cos (-π/4) = cos π/4 = 0,5√2