解答图中习题，要步骤

戴维宁等效电路是：等效电压U =UR3 -UR1 = 20[ 3/( 3 + 2 ) - 1/( 1 + 4 ) ] = 8v；等效电阻 R = 1//4 + 2//3 = 4/5 + 6/5 = 2Ω ；开关闭合，uc不能突变，uc(0+) = uc(0-) = 0；uc(∞) = U = 8v；ic(0+) = U/R = 8/2 = 4A；ic(∞) = 0；-t/τ = -t/(RC) = -t/( 0.05 * 2 ) = -10t；uc(t) =uc(∞) + [uc(0+) -uc(∞) ]e^(-t/τ)= 8 + [ 0 - 8 ]e^(-10t) = 8[ 1 -e^(-10t) ]ic(t) =ic(0+) + [ ic(0+) - ic(∞) ]e^(-t/τ)= 0 + [ 4 - 0 ]e^(-10t) = 4e^(-10t) 。