[tex]sina=-\frac{3}{5} \\\\cos^2a+sin^2a=1\\\\cos^2a=1-\frac{9}{25} =\frac{16}{25} \\\\cos a=-\frac{4}{5}, cos\ este\ in \ cadran\ 2[/tex]
[tex]sin2a=2sinacosa=2\times\frac{3}{5} \times \frac{4}{5} =\frac{24}{25}\\\\ cos2a=cos^2a-sin^2a=\frac{16}{25}-\frac{9}{25} =\frac{7}{25} \\\\tg2a=\frac{sin2a}{cos2a} =\frac{24}{7} \\\\cos4a=cos^22a-sin^22a=\frac{576}{625} -\frac{49}{625}=\frac{527}{625}[/tex]
