根据几何关系 r acsco 1=acot(π-O)=-acot0 dl acsc2 0d0 B= sin ado 4πaJ8 D 编csA-c风)》1  2 r = acsc d csc d 2 l = a l = acot( − ) = −acot 根据几何关系 (cos cos ) 4 1 2 0    −  = a I   = 2 1 sin d 4 0 θ a θ I B    I a I l  d r   P l