ITS SIMPLE USE BASIC TRIGO TO GET IT

1} FIRSTLY GET THE BENT FIG DRAWN IN UR BOOK(MARK THE POLES P1 AND P2)

2}HERE U SEE R=L(L IS HERE THE CURVE LINE EARLIER IT WAS THE SAME DIST B/W THE POLES)MAKE A LINE JOINING THE POLES NOW AN ISOCELES  IS FORMED NOW ITS ELEMENTARY .THE ANGLE B/W THE TWO CONGRUENT LINES( RADIUS) IS