4.11. cos α = −0,8, π/2 < α < π.

\(\cos\alpha=-0.8=-\frac{4}{5}\). Во II четверти sin α > 0:

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

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

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