Question

Найдите cos2x, если tgx=3/4.

Answer 1

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