[tex]\displaystyle\bf\\180^\circ < \alpha < 270^\circ \ \ \ \Rightarrow \ \ \ Sin\alpha < 0\\\\\\Cos\alpha =-\frac{4}{5} \\\\\\Sin\alpha =-\sqrt{1-Cos^{2} \alpha } =-\sqrt{1-\Big(-\frac{4}{5} \Big)^{2} } =-\sqrt{1-\frac{16}{25} } =\\\\\\=-\sqrt{\frac{9}{25} } =-\sqrt{\Big(\frac{3}{5} \Big)^{2} } =-\frac{3}{5}=-0,6\\\\\\Otvet \ : \ Sin\alpha =-0,6[/tex]