I = (Us - U) / R1 + Is
= (1 - U) / 1 + 2
= 3 - U (1)
U * I = 2 (2)
解得：
U = 2 V
I = 1 A
R = U / I = 2 Ω
http://wenku.baidu.com/view/1a26938d8762caaedd33d4bc.html
