Section 2.2: Flow of a Falling film using Table 3.4-2

vz is the only flow component and it depends only on x. Then eqs. D & E are trivial and eq. F
has only mu * the second derivative of vz wrt x + rho*the z component of gravity.
>  restart;
>  de:=mu*diff(vz(x),x,x)+rho*gz=0;
>  s:=dsolve({de,vz(delta)=0,D(vz)(0)=0},vz(x));
>  assign(s);vz:=unapply(vz(x),x);

>  vzbook:=x->rho*gz*delta^2*(1-(x/delta)^2)/(2*mu);
>  simplify(vz(x)-vzbook(x));