如图1所示电路的等效电阻Ｒab＝（　　）。 

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SQL Error: select id,body,options from ***_enewstiku_ where sourceid='68472' and id < 4420898 and id<>'4420898' order by id desc limit 5 SQL Error: select id,body,options from ***_enewstiku_ where sourceid='68472' and id > 4420898 and id<>'4420898' order by id asc limit 5