Найдите cos2x, если tgx=3/4.

[tex]cos2x= \frac{1-tg^2x}{1+tg^2x} = \frac{1- (\frac{3}{4})^2 }{1+ (\frac{3}{4})^2} = \frac{1- \frac{9}{16} }{1+9/16} = \frac{7}{16} * \frac{16}{25} = \frac{7}{25}[/tex]