# How do you evaluate arccos(-1/2)?

$\theta$= 120, 240 degrees
Let $\arccos \left(- \frac{1}{2}\right) = \theta , t h e n \cos \theta = - \frac{1}{2}$
It is known that cos 60 = $\frac{1}{2}$
Since $\cos \theta$ is negative in IInd and IVth quadrant, $\theta$ would be = 180-60 and 180+60, that is 120,240 degrees.