两支路并联，各支路电阻串联分压：
Uab = 48 * (6/8 - 3/6) = 12V
电压源短路，从 a、b 往两边电阻是并联：
Rab = 3//3 + 2//6 = 3Ω
I = Uab / (Rab + 13) = 0.75A
