find the value of cos (31pi/3) ?

w e k n o w t h a t \cos (2 k π + x) = \cos x N o w \cos (\frac{31 π}{3}) = \cos (10 π + \frac{π}{3}) = c o s (2 \times 5 π + \frac{π}{3}) = \cos \frac{π}{3} = \cos 60 = \frac{1}{2}