symsum(diff(f,x,k) * (x^k)/(factorial(k)),k,0,i)
is processed as
temp = diff(f,x,k) * (x^k)/(factorial(k));
symsum(temp, k, 0, i)
That is, the expression is evaluated first (as is the case for all MATLAB function arguments) and the result of the expression has the symsum applied to it.
But diff(f, x, k) with symbolic k is 0 . I do not know why they made that choice.
With it coming out as 0, you are doing symsum(0, k, 0, i) which is going to be 0.
I would advise not using symsum for this. Instead, keep a running total, adding one more term at each point.
