Remove["Global`*"]

par[list_List]:=((Total[(list^(-1))])^(-1));
ser[list_List]:=Total[list];

{R1,R2,R3,R4,R5}=Range[100,500,100.]

ser[{R1,par[{ser[{par[{R3,R4}],R5}],R2}]}]

{R1,R2,R3,R4,R5,R6,R7}=Range[100,700,100.]

ser[{R1,par[{R2,R3,ser[{R4,R5}]}],par[{R6,R7}]}]

iZ=10*10^-3;

rce={iR3==iR1+iZ,iZ==iR3+iR2+iout,
u1==iR1*R1,u2==R2*iR2,u2-u1==iR3*R3};

neznameAll=Cases[rce,_Symbol,{0,∞}]//Union
nezname=DeleteCases[neznameAll,iout]
Solve[rce,{iR1,iR2,iR3,u1,u2}]

Partition[IntegerDigits[Round[(10^5000)*Pi]],3,1];
Cases[%,{1,2,3}]
Position[%%,{1,2,3}]