Question

Explain the method of this.. Pic attached!

How we get pi/3 and 5pi/3 like that?

It would be great if you could help me!

Answer 1

the value of cos is positive in 1st and 4rth quadrant therefore its value is 1/2 if angle is
$\frac{\pi}{3} \: \: or \: \: (2\pi - \frac{\pi}{3} ) \\ \frac{\pi}{3} \: \: \: or \: \: \frac{5\pi}{3}$
if have any doubt please ask.