Question

Daca a apartine lui R tg a=2 , calculati cos 2a

Answer 1

Avem fomulele: $cos \alpha = \frac{1}{ \sqrt{1+tg^2 \alpha } } ,deci,cos \alpha = \frac{1}{ \sqrt{5} },apoi,cos2 \alpha=2cos^2 \alpha -1=2* \frac{1}{5}-1$ =-3/5