1) последовательное R1+R2=7Om
2) R1+R4=5
1/R(23)=1/R2+1/R3=1/4+1/3=7/12, т.е R(23)=12/7
R1+R4+R(23)=47/7
3) R1+R2+R3=12
4) 1/R3+1/(R1+R2)=1/4+1/13=17/52, т.е R=52/17
5) 1/R1+1/R2+1/R3=1/4+1/2+1/3=13/12. т.е R=12/13
6) R1+R2=6
1/R3+1/R4+1/R5=1/3+1/3+1/6=5/6, т.е R(345)=6/5
R1+R2+R(345)=6+6/5=36/5