Question

Срочно!!!!!!!!!!!!!!!!!!!

Answer 1

Ответ:

формула:

$cos \alpha cos \beta = \frac{1}{2} cos( \alpha - \beta ) + \cos( \alpha + \beta )$

$cos \frac{3x}{2} cos \frac{x}{2} = \\ = \frac{1}{2} \cos( \frac{3x}{2} - \frac{x}{2} ) \cos( \frac{3x}{2} + \frac{x}{2} ) = \\ = \frac{1}{2} cosx \: cos2x = \\ = \frac{1}{2}cosx \: ( {cos}^{2}x - {sin}^{2} x) = \\ \frac{1}{2}cosx \: ( {cos}^{2}x - (1 - {cos}^{2}x)) = \\ \frac{1}{2}cosx \: (2 {cos}^{2}x - 1)$

$cosx = \frac{2}{7}$

$cos \frac{3x}{2} cos \frac{x}{2} = \\ = \frac{1}{2} \times \frac{2}{7}(2 \times ( \frac{2}{7} )^{2} - 1) = \\ = \frac{1}{7} ( \frac{8}{49} - 1) = \frac{1}{7} \times ( - \frac{ 41}{49} ) = \\ = - \frac{41}{343} = - 0.12$