Question

Daca tg x/2 = 2, calculati cos x.

Answer 1

$cos x = \frac{1-tg^{2} \frac{x}{2} }{1+tg^{2} \frac{x}{2} } = \frac{1-4}{1+4} =- \frac{3}{5}$

Answer 2

cos x=(1-tg^2 x/2)/1+tg^2 x/2=(1-4)/1+4=-3/5