用戴维宁定理求出网络的等效电源与电阻:
Uab = 20 + 2 = 22 V
Rab = 6 + 4 = 10 Ω
R = 10 Ω 时获得最大功率 Pmax :
Pmax = 11 * 11 / 10 = 12.1 W
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