TeX джерело:

\cos \alpha = \pm \sqrt {1 - {{\sin }^2}\alpha } = \pm \sqrt {1 - {{( - \frac{4}{5})}^2}} = \pm \sqrt {1 - \frac{{16}}{{25}}} = \pm \sqrt {\frac{9}{{25}}} = \pm \frac{3}{5}