Question

Calculati cos 2a , stiind ca tg a/2= 2

Answer 1

$\displaystyle\cos a=\frac{1-\tan^2\frac{a}{2}}{1+\tan^2\frac{a}{2}}=-\frac{3}{5}$
$\displaystyle\cos 2a=2\cos^2a-1=-\frac{7}{25}$