Quiet@Remove["Global`*"];

U=750*(10^3);
R1=2;
R2=5;
Ptotal=1500*(10^6);
P1=(U-R1*i1)*i1;
P2=(U-R2*i2)*i2;
podminka=P1+P2==Ptotal;
ztraty=R1*(i1^2)+R2*(i2^2);
sol=NMinimize[{ztraty,podminka},{i1,i2}]

Lagrange=ztraty+λ*(podminka/.Equal[lhs_,rhs_]:>lhs-rhs);
rceLagrange=0.==D[Lagrange,#]&/@{i1,i2,λ};
solLagrange=Quiet@Solve[rceLagrange,{i1,i2,λ}]
{P1+P2,ztraty}/.solLagrange

{(U-R1*i1),ztraty}/.solLagrange[[2]]

Quiet@Remove["Global`*"];
Uz=230.;
ω=314;
R1=0.1;
L1=0.01;
R2=0.05;
L2=0.005;
Rz=10;
Lz=0.05;
Z1=R1+I*ω*L1;
Z2=R2+I*ω*L2+(1/(I*ω*c));
Zz=(1/((1/Rz)+(1/(I*ω*Lz))));
Zzz=((Zz*Z2)/(Zz+Z2));
i1=(Uz/(Z1+Zzz));
Uzatez=Uz-i1*Z1;
i2=(Uzatez/Z2);
ztraty=R1*Abs[(i1^2)]+R2*Abs[(i2^2)];
{ztratyMin,dosC}=NMinimize[ztraty,c]
{Uz*Conjugate[i1/.dosC],ztraty/.dosC,Abs[Uzatez/.dosC]}
{Uz*Conjugate[i1/.c->(10^(-12))],ztraty/.c->(10^(-12)),Abs[Uzatez/.c->(10^(-12))]}

NMaximize[Abs[Uzatez],c]
FindRoot[Abs[Uzatez]==Uz,{c,(10^(-4))}]
dosC

par:=((Total[((#)^(-1))])^(-1))&;
ser:=Total;
Zz=par[{Rz,I*ω*Lz}];
Zzz=par[{Z2,Zz}];