Example 10.5-2 for water comparing the approximation in eq. 10.5-19c 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);
>  mu:=proc(x)
>   local T1;global T, B;
>   T1:=T(x);
>   mu0*exp(B*((1/T1)-(1/T0)));
>  end:
>  A:=mu(delta)/mu0;
>  muapp:=proc(x)
>   global delta, mu, mu0, A;
>   mu0*A^(x/delta);
>   end:
Here are the values found for water from Table 1.1-1
>  B:=1868.6; mu0:=1.787; T0:=273.15; Td:=373.15;
>  de:=diff(tauxz(x),x)=rho*g*cos(beta);
>  s:=dsolve({de,tauxz(0)=0},tauxz(x)):
>  assign(s);
>  tauxz:=unapply(tauxz(x),x);
>  de2:=tauxz(x)=-muapp(x)*diff(vz(x),x); Note muapp is used here
>  s:=dsolve({de2,vz(delta)=0},vz(x)):
>  assign(s);
>  vz:=unapply(vz(x),x);
>  de3:=tauxz(x)=-mu(x)*diff(vz3(x),x); Note mu 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.

>