Question

$\cos( \frac{4\pi}{5} )$

Answer 1

[tex]\cos\left(\dfrac{4\pi}{5}\right) =\cos\left(\pi -\dfrac{\pi}{5}\right)=-\cos \dfrac{\pi}{5} \\ \text{Din monotonia functiei cosinus avem ca:}\\ \cos \dfrac{\pi}{2}\ \textgreater \ \cos \dfrac{\pi}{5} \ \textgreater \ \cos \pi\\ 0\ \textgreater \ \cos \dfrac{\pi}{5} \ \textgreater \ -1\\ \text{Asadar:} \boxed{\left[\cos \dfrac{\pi}{5} \right]=-1}[/tex]