求解，要画等效图

11a、Rab = ( 6//3 + 2 )//4 + 10 =( 2 + 2 )//4 + 10 = 2 + 10 = 12Ω；11b、Rab = ( 10//10 + 5 )//10 + 5 = ( 5 + 5 )//10 + 5 = 5 + 5 =10 Ω；11c、Rab = [ ( 4//4 + 2 )//4 + 4//4 ]//4=[ ( 2 + 2 )//4 + 4//4 ]//4 =[ 2 + 2 ]//4 = 2Ω；等效电阻的图就是一只电阻。