电路如图所示，电阻R可变，为使R获得最大功率，R应为（   ）。

试卷相关题目

SQL Error: select id,body,options from ***_enewstiku_ where sourceid='69470' and id < 4499516 and id<>'4499516' order by id desc limit 5 SQL Error: select id,body,options from ***_enewstiku_ where sourceid='69470' and id > 4499516 and id<>'4499516' order by id asc limit 5