Question

urgente!!!!!!!!!!!!!
se sen(x)=-2/3, cos(2x)sen(-x) é?

Answer 1

$\cos(2x)=1-2\,\mathrm{sen^2}(x)\\\\ \mathrm{sen}(-x)=-\,\mathrm{sen}(x)$


Portanto,

$\cos(2x)\cdot \mathrm{sen}(-x)\\\\ =\big(1-2\,\mathrm{sen^2}(x)\big)\cdot \big(-\mathrm{sen}(x)\big)\\\\ =\left(1-2\cdot \left(-\dfrac{2}{3} \right )^{\!\!2}\right)\cdot \dfrac{2}{3}\\\\\\ =\left(1-2\cdot \dfrac{4}{9}\right)\cdot \dfrac{2}{3}\\\\\\ =\left(1-\dfrac{8}{9}\right)\cdot \dfrac{2}{3}\\\\\\ =\left(\dfrac{9}{9}-\dfrac{8}{9}\right)\cdot \dfrac{2}{3}\\\\\\ =\dfrac{9-8}{9}\cdot \dfrac{2}{3}\\\\\\ =\dfrac{1}{9}\cdot \dfrac{2}{3}\\\\\\ =\dfrac{2}{27}$