Question

cosx=3/5 x Э(3П/2П) ,вычислить cos 2x

Answer 1

$\cos(2x) = \cos^2(x) - \sin^2(x) = \cos^2(x) - (1 - \cos^2(x)) = 2\cos^2(x) - 1$

$\cos(2x) = 2\cdot (\frac{3}{5})^2 - 1 = 2\cdot\frac{9}{25} - 1 = \frac{18}{25} - 1 = \frac{18-25}{25} = -\frac{7}{25}$