Off[General::"newpkg",General::"obspkg"]
<<Calculus`VectorAnalysis`;
Off[General::"spell1"];

SetCoordinates[Cartesian[x,y,z]];
d=0.9;
λ=0.8;
α1=5;
α2=17;
Tuvnitr=20;
den=24*3600;
Tvenku=10+7*Sin[((2*Pi*t)/den)];
ro=1800;
cp=1000;
rocp=ro*cp;

resx=Solve[x==1]
x/.resx[[1]]

x+3y/.x->zaba

Plot[(t/(t+1)),{t,0,100},PlotRange->All]

Options[Plot3D];

D[T[x,t],{x,3},{t,2}]

Tpoc=10;
tmax=6*den*((((d/0.6)))^2);
rce=rocp*D[T[x,t],t]==Div[λ*Grad[T[x,t]]];
poc=T[x,0]==Tpoc;
okrVlevo=α1*(Tuvnitr-T[x,t])*(t/(t+1))==-λ*D[T[x,t],x]/.x->0;
okrVpravo=α2*(T[x,t]-Tvenku)*(t/(t+1))==-λ*D[T[x,t],x]/.x->d;
res=NDSolve[{rce,poc,okrVlevo,okrVpravo},{T[x,t],D[T[x,t],x]},{x,0,d},{t,0,tmax}];
vysledek=T[x,t]/.res[[1]];
derivace=Derivative[1,0][T][x,t]/.res[[1]];
Plot3D[vysledek,{x,0,d},{t,0,tmax},AxesLabel->{"x[m]","t[s]","T[°C]"},PlotPoints->30];
tokuvnitr=-λ*derivace/.x->0;
Plot[tokuvnitr,{t,0,tmax},AxesLabel->{"t[s]","q(W/(m^2))"},PlotRange->{Automatic,{0,15}}]

((((3.^(1/3))))^2)