Question

Помогите найти:

tg(a/2), если cos(a) = 4/5

3пи/2 < а < 2пи

Answer 1

tg^2a/2=(1-cosa)/(1+cosa)=
(1-4/5)/(1+4/5)=1/5/9/5=1/9
3π/2<a<2π
3π/4<a/2<π
tga/2=-1/3