Question

Найти cos2a если tga= -0.2

Answer 1

$\cos 2x = 2\cos^2x - 1 = \dfrac{2}{\mathrm{tg}^2x + 1} - 1 = \dfrac{2}{0{,}04 + 1} - 1 = \dfrac{0{,}96}{1{,}04} = \dfrac{12}{13}$