从右往左推，
U 2 ( s ) = 1 C 2 s I 2 ( s ) U_2(s)=\frac 1{C_2s}I_2(s) U2(s)=C2s1I2(s)
I 2 ( s ) = 1 R 2 [ U 3 ( s ) − U 2 ( s ) ] I_2(s)=\frac 1{R_2}[U_3(s)-U_2(s)] I2(s)=R21[U3(s)−U2(s)]
U 3 ( s ) = 1 C 1 s [ I 1 ( s ) − I 2 ( s ) ] U_3(s)=\frac 1{C_1s}[I_1(s)-I_2(s)] U3(s)=C1s1[I1(s)−I2(s)]
I 1 ( s ) = 1 R 1 [ U 1 ( s ) − U 3 ( s ) ] I_1(s)=\frac{1}{R_1}[U_1(s)-U_3(s)] I1(s)=R11[U1(s)−U3(s)]
