Solution (source code)

= Solution

Write the remainders after order $N$ as $r_N=o((x-x_0)^N)$ for $f$ and $s_N=o((x-x_0)^N)$ for $g$. Then
$$
f(x)+g(x)-\sum_{n=0}^N(f_n+g_n)(x-x_0)^n
=r_N(x)+s_N(x)
=o((x-x_0)^N).
$$
\b[Hence $h=f+g$ has the termwise sum of the two <asymptotic expansions>.]