Вначале упростим выражение: (1- cos2a+sin2a)/(cosa - sin(2pi-a))
sin(2pi - a) = sin(-a) = -sin(a)
cos(2a) = 1 - 2sin^2(a)
sin(2a) = 2*sin(a)*cos(a)
(1 - 1 + 2sin^2(a) + 2sin(a)*cos(a)) / (cos(a) + sin(a)) = 2sin(a)*(sina(a) + cos(a)) / (sin(a) + cos(a)) = 2sin(a)
при sin(a) = -1/2, 2sin(a) = 2*(-1/2) = -1
Ответ: -1.