U=U1=I1R1=0.5*20=10V
S2断开S1S3闭合，R1R2并联
I=I1+I2
0.9=0.5+I2
I2=0.4A
R2=U/I2=10/0.4=25Ω
只闭合S2，R1R2串联
U1/U2=R1/R2
U1/(10-U1)=20/25
U1=4.4V
如图所示的电路中，电源电压不变，电阻R1的阻值为20欧。当断开开关S1和S2，闭合开关S3时，电流
S1S2断薯漏开握模，S3闭合，电路只有R1工段手缓作U=U1=I1R1=0.5*20=10VS2断开S1S3闭合，R1
U=U1=I1R1=0.5*20=10V
S2断开S1S3闭合，R1R2并联
I=I1+I2
0.9=0.5+I2
I2=0.4A
R2=U/I2=10/0.4=25Ω
只闭合S2，R1R2串联
U1/U2=R1/R2
U1/(10-U1)=20/25
U1=4.4V
U=U1=I1R1=0.5*20=10V
S2断开S1S3闭合，R1R2并联
I=I1+I2
0.9=0.5+I2
I2=0.4A
R2=U/I2=10/0.4=25Ω
只闭合S2，R1R2串联
U1/U2=R1/R2
U1/(10-U1)=20/25
U1=4.4V