Series[Exp[r],{r,0,5}]

Rt1=(1/(2*Pi*λ*delka))*Log[(r2/r1)];
S=2*Pi*r2*delka;
Rt2=(1/(α*S));
Rtsum=Rt1+Rt2;

dosIzolujeme={λ->0.04,r1->0.15,delka->1,α->15};
cena1kubik=2;
cenaizolace=cena1kubik*Pi*delka*((r2^2)-(r1^2))/.dosIzolujeme;
RtIzolujeme=Rtsum/.dosIzolujeme
Plot[{RtIzolujeme,cenaizolace},{r2,0.15,1.5},AxesLabel->{"r[m]","Rth[K/W]"},PlotStyle->Thick]

dosChladime={λ->0.3,r1->0.0005,delka->1,α->15};
Solve[(Simplify[D[Rtsum,r2]]//Numerator)==0,r2][[1]]
Print["polomer extremu= ",rextr=(r2/.%)/.dosChladime," m"]
RtChladime=Rtsum/.dosChladime;
Plot[RtChladime,{r2,rextr-0.01,rextr+0.01},AxesLabel->{"r[m]","Rth[K/W]"},PlotStyle->Thick]