Если cosa=-1/3 и 180

Cos (a) = cos^2 (a/2) - sin^2 (a/2)

cos^2 (a/2) + sin^2 (a/2) = 1

2*cos^2 (a/2) = cos (a) + 1 = 2/3

cos^2 (a/2) = 1/3, sin^2 (a/2) = 2/3

{90 < a < 135}

cos (a/2) = - sqrt (1/3)

sin (a/2) = sqrt (2/3)

tg (a/2) = - sqrt (2)