Horse Grazing

A horse is tethered by a rope fastened at one end $A$ on the boundary of a circular field whose center is $O$ . The horse can graze over $\dfrac { 1 } { 7 }$ the part of the field reaching the farthest point $B$ on the boundary that he can reach. If $\angle AOB = \theta$ . Find the value of $\dfrac { 7(\sin { \theta +\left( \pi -\theta \right) \cos { \theta } ) } }{ \pi }.$