0\\\\sin\alpha=-\frac{4}{5}\\\\sin^2\alpha+cos^2\alpha=1\\\\\left(-\frac{4}{5}\right)^2+cos^2\alpha=1\\\\\frac{16}{25}+cos^2\alpha=1\\\\cos^2\alpha=1-\frac{16}{25}\\\\cos^2\alpha=\frac{9}{25}\\\\cos\alpha=-\sqrt\frac{9}{25}\\\\cos\alpha=-\frac{3}{5}" alt="\alpha\in(180^o;\ 270^o)\to sin\alpha;\ cos\alpha < 0\ \wedge\ tg\alpha;\ ctg\alpha > 0\\\\sin\alpha=-\frac{4}{5}\\\\sin^2\alpha+cos^2\alpha=1\\\\\left(-\frac{4}{5}\right)^2+cos^2\alpha=1\\\\\frac{16}{25}+cos^2\alpha=1\\\\cos^2\alpha=1-\frac{16}{25}\\\\cos^2\alpha=\frac{9}{25}\\\\cos\alpha=-\sqrt\frac{9}{25}\\\\cos\alpha=-\frac{3}{5}" align="absmiddle" class="latex-formula">
