Example 11.4-3 for water comparing the approximation in eq. 11.4-19 to the more accurate expression for the viscosity.
>  restart;
>  s:=dsolve({diff(T(x),x,x),T(0)=T0,T(delta)=Td},T(x)):
>  assign(s);
>  T:=unapply(T(x),x);
>  mu17:=x->mu0*exp(B*(1/T(x)-(1/T0)));
>  mud:=mu17(delta);
>  A:=mud/mu0;
>  mu19:=x->mu0*A^(x/delta);
Here are the values found for water from Table 1.1-2
>  B:=1868.6; mu0:=1.787; T0:=273.15; Td:=373.15;
>  de:=diff(tauxz(x),x)=rho*g*cos(beta); Eq. 2.2-8
>  s:=dsolve({de,tauxz(0)=0},tauxz(x)):
>  assign(s);
>  tauxz:=unapply(tauxz(x),x); This agrees with 2.2-13
>  de2:=tauxz(x)=-mu19(x)*diff(vz(x),x); Note mu19 is used here in eq. 2.2-14 with mu
as a function of x.
>  s:=dsolve({de2,vz(delta)=0},vz(x)):
>  assign(s);
>  vz:=unapply(vz(x),x);
>  de3:=tauxz(x)=-mu17(x)*diff(vz3(x),x); Note mu17 is used here.
>  s:=dsolve({de3,vz3(delta)=0},vz3(x)):
>  assign(s);vz3:=unapply(vz3(x),x);
>  beta:=Pi/4; delta:=.2; rho:=1; g:=980.665;
>  vz(x);
>  evalf(vz(x));
>  evalf(vz(0));evalf(vz3(0));
>  evalf(vz(delta));
>  evalf(vz3(x));
>  plot([vz(t*delta),vz3(t*delta)],t=0...1,color=[red,green]);
Green is the more accurate expression for viscosity as a function of position.

>