function  a=pqfile(n)
fp=fopen('wheredata.m')
disp(',reading p and q of  file p in a line then q in a line,')
syms x p q y a
for k=1:n, 
   disp('======================problem  number ='),
   disp(k),
   p = fgetl(fp);
   q = fgetl(fp);
   y=linode1(p,q),
   % if another program is needed such as exact1 then delete the previou line
   % and add y=exact1(p,q)   where p=m and q=n
end;
fclose(fp);
return
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%file wheredata.m  has  four problems below:
x 
3*x
1/x
3/x
4
-2*x
tan(x)
cos(x)
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
function y=linode1(p,q)
% - revision feb 12 2004(RMM)
% - Linear nonhomogeneous  first order ode
% - usage y=linode1(p,q)  to solve  y'+p y =q.
% - some authors like to call the integrating factor a source or Green function.
      syms x u yh yp y c
      disp(' you are solving  yprime+py=q')
      disp(' where p = '); disp(p)
      disp(' and, where q = '); disp(q)      
      disp('Integrating Factor u= exp(int(p))') 
      u=simplify(exp(int(p,'x')))
      disp(' Homogeneous solution is  yh=c/u') 
      yh=c/u  
      disp(' The Particular solution is yp=1/u * Int( u*q)') 
      yp=simplify(int(u*q,'x')/u)
      disp(' The general  solution is y = yh+yp')
      y=simplify(yh+yp)
      disp(' compare with output from the  dsolve routine in matlab')
      dsolve('Df=p*f+q','x')  
      return