按规则断开 R1 ，求 Uab ：
Uab = Us + Is * R2 = 20 + 10 * 4 = 60 V
Us 短路、Is 开路，求 Rab ：
Rab = R2 = 4 Ω
IR4 = Uab / (Rab + R4) = 6 A
U = IR4 * R4 = 36 V
