How to prove that $\max(f(n), g(n)) = \Theta(f(n) + g(n))$?

Using the basic definition of theta notation prove that $\max(f(n), g(n)) = \Theta(f(n) + g(n))$

I came across two answer to this question on this website but the answers weren't clear to me. Would you mind to elaborate how this can be proven? I am first year student of computer sciences. Thank you!

Edit:

What exactly does $\max(f(n), g(n))$ return?

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It's not true in general without some condition, the most obvious being that both are positive functions. –  Thomas Andrews Dec 29 '12 at 19:46

Note that $f(n) \leq f(n) + g(n)$ and $g(n) \leq f(n) + g(n)$. Hence, $$\max(f(n), g(n)) \in \mathcal{O}(f(n) + g(n))$$ Next note that $f(n) + g(n) \leq 2 \max(f(n),g(n))$. Hence, $$\max(f(n), g(n)) \in \mathcal{\Omega}(f(n) + g(n))$$ Hence, we get that $$\max(f(n), g(n)) \in \mathcal{\Theta}(f(n) + g(n))$$
Note that $$\max(f(n),g(n)) = \begin{cases} f(n) & \text{if } f(n) \geq g(n)\\ g(n) & \text{if } g(n) \geq f(n) \end{cases}$$ For instance, if $f(n) = 10n$ and $g(n) = n^2$, we get that $$\max(f(n),g(n)) = \begin{cases} 10n & \text{if } n \leq 10\\ n^2 & \text{if } n \geq 10 \end{cases}$$
@Peter For a fixed $n$, if $f(n) \geq g(n)$, then we have that $f(n) +f(n) \geq f(n) + g(n) \implies f(n) + g(n) \leq 2 f(n)$ and if $g(n) \geq f(n)$, then we have that $g(n) +g(n) \geq f(n) + g(n) \implies f(n) + g(n) \leq 2 g(n)$. Hence, we get that $$f(n) + g(n) \leq 2 \max(f(n) + g(n))$$ The key observation is that $f(n) \leq \max(f(n),g(n))$ and $g(n) \leq \max(f(n),g(n))$ and hence $$f(n) + g(n) \leq 2 \max(f(n) + g(n))$$ –  user17762 Dec 29 '12 at 19:49
For instance, if we have $f(n) \geq g(n)$ adding $f(n)$ to both sides, we get that $2f(n) \geq f(n) + g(n)$ and similarly for the other case as well. –  user17762 Dec 29 '12 at 19:56