+R∥R A f 1+A ra+R∥RR+Rt R∥R) R+Re R+Re Rrx'∥(R+R)5∥(R+R).l+R∥R,R+R R R R∥RR+Rf 若F>>R∥R,r<<(R+R1,则 R1 R。≈r A≈A AF≈A R+Ro 1+A R+Re R rid (+ A Aodd r Re r+r R+Re 1+A R+Re29 R R R r r R R A r R R R R R r R R r A r R R R R R r R A R R R r R R r R r R R A R R R r R R r A r R R r A A f id id f o d o f id f f id o d o f o f f id o d id f f id if id f o d id f f id o d id f id o d f ( ) ( ) ( ) 1 +  +  + = +  +  +  + =  +  +  +  +  + +  +  = ∥ ∥ ∥ ∥ ∥ ∥ ∥ ∥ R R R A r R R R A r R R R A r R R R R R r A R R R A A A R R R A A AF A R r R r r R R r R R f o d o f o d o o f f o d id f if id o d f o d o d f f o d o d i id o o id f o f 1 (1 ) 1 ( ) +   + +   +  +  +  + +   +      若 ＞＞ ∥ ， ＜＜ + ，则