就两个回路的求解而已
I1 = (10 - 5) / (5+5) = 0.5A
Uab = 5 + 0.5 * 5 + 4 = 11.5V
Rab = 5//5 + 2 = 4.5Ω
I = 11.5 / (4.5 + 2) ≈ 1.77A
