3．开关动作前电路已处于稳定状态，用三要素法求开关动作后的iL(t)。

一、v4 = 2.4/0.4 = 6v；I3 = -3.6/6 = -0.6A；v1 = 12v - v4 = 6v；P2 = v1 * ( 0.4 + I3 ) = 6( 0.4 + 0.6 ) = 6w；二、列节点电压方程：V1( 1 + 1/2 ) - V2/1 - 3/2 = -2，3V1 - 2V2 = -1；①-V1/1 + V2( 1/2 + 1 + 1/0.5 ) - 3/0.5 = 0，-2V1 + 7V2 = 12；②① * 2 + ② * 3，17V2 = 34，V2 = 2v；3V1 = -1 + 2 * 2，V1 = 1v；Vx = V2 - V1 = 2 - 1 = 1v 。三、开关转换，iL不能突变。iL(0-) =iL(0+) = 6 * 1/( 1 + 2 ) = 2A；iL(∞) = 6/2 = 3A；-t/τ = -t/[ L/R ] = -t/[ 1/(2//2) ] = -t；iL(t) =iL(∞) + [iL(0+) -iL(∞) ]e^(-t)= 3 + [ 2 - 3 ]e^(-t)= 3 -e^(-t) A 。