选坐标如图示 x-l=rtg B R dB sin B R sin B [R2+( R B 1 R2In·dl [R2+(x-l)      2 1 2 3 2 2 2 [ ( ) ] 2 L L o R x l R In dl B  选坐标如图示 R x l R R  3 2 3 2 2 2 sin [ ( ) ]    x  l  Rctg    d R dl 2 sin  1  2  p l x R o I dl  B  I I