Если ctg a = 3 то cos (2a-5 пи) равен?

Cos (2a-5pi) = - cos2a=1-2cos²a=1-2 / (1+tg²a) tga=1/ctga=1/3 cos (2a-5pi) = 1-2*9/10=1-9/5=-4/5

Ctg a = 3; ctg²a = 9; sin²a = 1 / (1 + ctg²a) = 1 / (1 + 9) = 1/10 cos (2a-5 пи) = cos (-5 пи + 2a) = cos (5 пи - 2a) = - cos2a = - (1 - 2sin²a) = 2sin²a - 1 = 2*1/10 - 1 = 2/10 - 1 = 1/5 - 1 = - 4/5