左边闭合回路电流：
I1 = (12-6) / (3+6) = 2/3 A
Uab = 6 + 3 * 2/3 - 2 * 1 = 6 V
Rab = 3 // 6 + 1 +1 = 4 Ω
I = 6 / (4+2) = 1 A