Известно, что ctg t = - 20/21, 3 П/2 < t < 2 П

Ctg (П/2+t) = 2V6

-tg t=2V6

tg t=-2V6

a) cos (3 П/2-t) = - sin t

tg^2 t+1=1/cos^2 t

Cos^2 t=1 / (tg^2 t+1)

Cos^2 t=1 / ((-2V6) ^2+1) = 1/25

Cos t=V (1/25) = - 1/5

Sin t=V (1-cos^2 t) = V (1 - (-1/5) ^2) = V (1-1/25) = V (24/25) = 2V6/5

Cos (3 П/2 - t) = - sin t = (-2V6) / 5

б) cos (2 П-t) = cos t=-1/5