4.9. cos α = −3/5, π/2 < α < π.

Во II четверти sin α > 0.

\(\sin\alpha=\sqrt{1-\frac{9}{25}}=\frac{4}{5}\).

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

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