A simple pendulum of length l has a maximum angular displacement θ...

Here : length of pendulum $=l$ maximum angular displacement $=\theta$ mass of the bob $=m$ Now height of the bob at maximum angular displacement is $h=l-l \cos \theta=l(1-\cos \theta)$ Also at the end of the displacement kinetic energy of the bob = potential energy of the bob $=m g h=m g l(1-\cos \theta)$