i is 电压u2的叠加 R2 us 4+ RR2 n is R+R2 1+R2 (a) =u2+u2 R ↓ (b) 4=46-0- R+R2 ”R1 + + 4,=4e RR R+R (c)+ ' " u u i R R R R u R R R u 2 2 S 1 2 1 2 S 1 2 2 2       S 1 2 2 0 2 2 S u R R R u u i '     S 1 2 1 2 0 2 2 S i R R R R u u u "     电压u2的叠加