Question

Pronto, para indicar os valores

Answer 1

a) $\cos \dfrac{\pi}{2}=0$


b) $\cos 2\pi=1$


c) $\cos \dfrac{5\pi}{2}$

$=\cos \left(\dfrac{\pi+4\pi}{2} \right )\\ \\ =\cos \left(\dfrac{\pi}{2}+\dfrac{4\pi}{2} \right )\\ \\ =\cos \left(\dfrac{\pi}{2}+2\pi \right )\\ \\ =\cos \dfrac{\pi}{2}\\ \\ =0$