节点电压法：
(1/12 + 1/6 + 1/3) * Uab = 42/12 + 7
Uab = 18V
I1 = (42 - 18) / 12 = 2A
I2 = - 18/6 = -3A
I3 = 18/3 = 6A
网页链接
