★统一积分变量 l=actg(π-a)=-actga dl a csc'ada r a/sina B=∫Im@ )4π 2 2 sin ada I sina 2 sin a -ma1s咖a (c0sz1-c0sa2） B cosa-cosa? 4元4统一积分变量 a dl P  l 1  2 r dB  I O X Y l = actg( −) = −actg dl a csc d 2 = r = a sin =        2 2 2 0 4 sin ad I sin a sin =  2 0 4 r I sin dl B    =  2 1 sin 4  0      I d a (cos cos ) 4 1 2 0     = − a I B (cos cos ) 4 1 2 0     = − a I