Mạch: `R_1////[R_2  nt  (R_3////R_4)]`

`a)`

`R_[34]=[R_3R_4]/[R_3+R_4]=[10*10]/[10+10]=5Omega`

`R_[234]=R_2+R_[34]=10+5=15Omega`

`R=[R_1R_[234]]/[R_1+R_[234]]=[15*15]/[15+15]=7.5Omega`

`b)`

`U_1=U_[234]=U`

`I_1=U_1/R_1=U/15`

`I_[234]=[U_[234]]/[R_[234]]=U/15`

`I_2=I_[34]=I_[234]=U/15`

`U_[34]=I_[34]R_[34]=U/15*5=U/3`

`U_3=U_4=U_[34]=U/3`

`I_3=U_3/R_3=[U/3]/10=U/30`

Có `I_A+I_3=I_1`

`-> I_1-I_3=3`

`-> U/15-U/30=3`

`-> U=90V`

Khi đó `I_1=90/15=6A`, `I_2=90/15=6A`, `I_3=90/30=3A`, `I_4=90/30=3A`