弥尔曼定理
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转换成电流源..... VX电压 = 电流源和*电阻的并联和
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i1= 5/2
i2=10/2
i3=15/2
i4=20/4
电阻并联= 1(1/2+1/4+1/2+1/4)
i总*R并 = 17.5*2/3 =11.7V