Background: I'm studying roots of complex variables (i.e. not homework!), and going through a worked problem from Schaum's Outlines on Complex Variables.
In a worked problem, the following equation is presented and assumed the reader knows trig well enough: $\cos\frac\theta2 = \pm\sqrt{\frac{(1 + \cos \theta)}{2}}$
Can someone show me (or just hint) why this equation is true please?