SORRY I WAS WRONG NOW I GOT IT SEE FIRSTLY M=ml where m=strength l =distance b/w two poles. now earlier M=ml(bar magnet) now it is bent so l= r

 where r is the radius of the arc now to calc the new M we shud know the distance b/w the poles now the dist will be 2l/sin(/2) (u can calc it by simple geometry ) here note that if  is small then then M(earlier) =M(LATER) 

HOPE ITS CLEAR RATE ME IF U ARE SATISFIED 

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