如图，断开 R3 后，左、右是两个独立的闭合回路：
Uab = Us / 2 - Is * R4 = - 1O V ； R1 = R2
Rab = R1 // R2 + R4 = 9 Ω
I = Uab / (Rab + R3) = - 1 A
http://wenku.baidu.com/view/1a26938d8762caaedd33d4bc.html
