Solution
= Solution
By <Big O notation>, there are $C_1,C_2>0$ and thresholds after which $f_1(n)\leq C_1g_1(n)$ and $f_2(n)\leq C_2g_2(n)$. Choose a common threshold and set $C=\max(C_1,C_2)$. The positivity assumption gives
$$
0\leq f_1(n)+f_2(n)\leq C(g_1(n)+g_2(n)).
$$
Squaring preserves this inequality, so
$$
\boxed{(f_1+f_2)^2\leq C^2(g_1+g_2)^2\text{ eventually},\qquad(f_1+f_2)^2=O((g_1+g_2)^2).}
$$