Texternal = -10;
Tin = 26;
αin = 5;
αext = 15;
S = 4*18*4;
d = 0.45;
λ = 0.7;
Rt1 = (1/(αext*S));
Rt3 = (1/(αin*S));
Rt2 = (d/(λ*S));
Q = ((Tin - Texternal)/(Rt1 + Rt2 + Rt3));
Print["ztraty jsou ", 
      (Q/1000), "  kW"]

resT1 = Solve[Q == ((Tin - T1)/Rt3)][[1]]

resT2 = Solve[Q == ((T2 - Texternal)/Rt1)][[1]]

Rtpuvodni = Rt1 + Rt2 + Rt3;
rtpuvodni = Rtpuvodni*S;
\(Print["rt puvodni= ", 
      rtpuvodni, \*""  \!\(\(\(m\^2\) K\)\/W\)""]\)

λiz = 0.04;
Solve[rtpuvodni + (x/λ)iz == 3]

xiz = 0.1;
rnove = 
      rtpuvodni + (x/λ)iz /. x -> xiz

Print["usetrime cca  ", 
      Round[100 - 
          100*(rtpuvodni/rnove)], "% energie na prostup"]

qnove = ((Tin - Texternal)/rnove);
Trozhrani1 = 
        Tin - qnove*((xiz/λ)iz + 
                (1/α)in);
Print["Trozhrani1= ", Trozhrani1, "  °C"]

Trozhrani2 = 
        Texternal + 
          qnove*((xiz/λ)iz + 
                (1/α)ext);
Print["Trozhrani2= ", Trozhrani2, "  °C"]

ClearAll[λiz, λ, r1, 
        r2, α];
Rtheta = (1/(2*Pi*el))*Log[(r2/r1)]

Series[Exp[x], {x, 0, 6}]

Plot[{Log[x], 0.01 ((x^2) - 1)},{x, 1, 20},PlotStyle -> {{Hue[0], Thickness[0.01]}, {GrayLevel[0], 
                Thickness[0.01]}},AxesLabel->{"x","ln[x],c[x]"}];

vyr = 
        Log[(r2/r1)]/(2  Pi*λ) + 
          (1/(2  Pi*r2*α));
resr2 = Solve[D[vyr, r2] == 0, r2][[1]]

D[vyr, {r2, 2}] /. resr2

(0.04/15)