restart;eq:=diff(T(t),t)=-k*(T(t)-Tm);
>

                          d
                    eq := -- T(t) = -k (T(t) - Tm)
                          dt

> cond:=T(0)=T0;
>

                          cond := T(0) = T0

> sol:=dsolve({eq,cond},{T(t)});
>

               sol := T(t) = Tm + exp(-k t) (-Tm + T0)

> tt:=unapply(rhs(sol),t,Tm,T0,k);

          tt := (t, Tm, T0, k) -> Tm + exp(-k t) (-Tm + T0)

> plot(tt(t,20,10,0.1),t=0..100);