Если 270 < a < 360, то sin a < 0, cos a > 0.
cos a = 3√11/10, cos^2 a = 9*11/100 = 99/100
sin^2 a = 1 - cos^2 a = 1 - 99/100 = 1/100
sin a = - √(1/100) = -1/10 = -0,1
