4.4. sin α = −0,8, π < α < 3π/2.

\(\sin\alpha=-0.8=-\frac{4}{5}\). В III четверти cos α < 0:

\(\cos\alpha=-\sqrt{1-\frac{16}{25}}=-\frac{3}{5}\).

\( g\alpha=\frac{4}{3}\), \(\ctg\alpha=\frac{3}{4}\).

Ответ: cos α = −3/5; tg α = 4/3; ctg α = 3/4.