1. R23=R2+R3=6+6=12 Ом;
R45=R4+R5=5+7=12 Ом;
R2345=(R23*R45)/(R23+R45)=(12*12)/(12+12)=144/24=6 Ом;
R=R1+R2345=1+6=7 Ом;
I=Uab/R=42/7=6 A.
2. R12=(R1*R2)/(R1+R2)=(3*6)/(3+6)=18/9=2 Ом;
R123=R12+R3=2+2=4 Ом;
1/R=(1/R123)+(1/R4)+(1/R5)=(1/4)+(1/12)+(1/6)=(6/24)+(2/24)+(4/24)=12/24;
R=24/12=2 Ом;
I=Uab/R=18/2=9 A.