cos²a = 1/5 -> sin²a = 4/5
tg²a = sin²a / cos²a = 4
cos2a = cos²a - sin²a = -3/5
sin(30) = 1/2
10 * (6/5 - 3/5) / (1/2 * 4) = 3
cos²a = 1/5 -> sin²a = 4/5
tg²a = sin²a / cos²a = 4
cos2a = cos²a - sin²a = -3/5
sin(30) = 1/2
10 * (6/5 - 3/5) / (1/2 * 4) = 3