Taylor Series expansion error

Question

PJS KUMAR 2018-9-29

0
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https://ww2.mathworks.cn/matlabcentral/answers/421366-taylor-series-expansion-error

回答： Walter Roberson 2018-9-29

I have the following code for taylor series expansion.

    function [sol]=Taylor(fn,a,b,y0,h,nd)
      syms x y(x);
       xd = a:h:b;
       yd(1) = y0;
       for i = 1:numel(xd)-1
       fstart= fn;
       fold = fstart;
       D(1) = subs(fold,{x,y},{xd(i),yd(i)});
       for j=2:nd
         fnew = subs(diff(fold,x),diff(y(x),x),fstart);
         D(j) =  subs(fnew,{x,y},{xd(i),yd(i)});
         fold = fnew;
       end
           yd(i+1) = yd(i)+h*D(1)+h^2/2*D(2)+h^3/6*D(3)+h^4/24*D(4)+h^5/120*D(5);
       end
      sol=[xd' yd'];
      end

It is giving correct output in one system. But giving the following errors in some other systems. Suggest me the solution to get the output.

>> syms x y(x)
>> f=x+y
f(x) =
x + y(x)
>> taylor(f,1,3,0.8,0.5,5)
Error using sym/taylor (line 99)
The value of 'x' is invalid. It must satisfy the function: @(x)isvector(x)&&isAllVars(x).