解二:dq=a·2m·d R dad gerar 2丌 dp=nrdi= otardr P=∫dP=了 grard="oo(R-R") OOT(R,-R(R,+R) 4 (R2+R 解一错误,解二正确! r R1 R2 解二： dq = 2rdr r r q I d 2 d d    = = P r I r r d m d d 2 3 =  = ( ) 4 2 1 2 R2 R q P m = +    4 ( )( ) ( ) 4 d d 2 1 2 2 2 1 2 2 4 1 4 2 3 2 1 R R R R P P r r R R R R m m − + = =  =  = −      解一错误，解二正确！