If $d(n)$ is $O(f(n))$ and $e(n)$ is $O(g(n))$, what is the order of $d(n) \times e(n)$? Please prove your answer.
2. If $\times$ means regular multiplication, I believe you are supposed to prove that $d(n)e(n)$ is $O(f(n)g(n))$.