<s>[INST] How to prove an inequality using mathematical induction? I have to prove the following:
$$ \sqrt{x_1} + \sqrt{x_2}  +...+\sqrt{x_n} \ge \sqrt{x_1 + x_2 + ... + x_n}$$
For every $n \ge 2$ and $x_1, x_2, ..., x_n \in \Bbb N$
Here's my attempt:
Consider $P(n): \sqrt{x_1} + \sqrt{x_2}  +...+\sqrt{x_n} \ge \sqrt{x_1 + x_2 + ... + x_n}$
$$P(2): \sqrt{x_1} + \sqrt{x_2} \ge \sqrt{x_1 + x_2}$$
$$ x_1 + x_2 + 2\sqrt{x_1x_2} \ge x_1 + x_2$$
Which is true because $2\sqrt{x_1x_2} > 0$.
$$P(n + 1): \sqrt{x_1} + \sqrt{x_2} + ... + \sqrt{x_n} + \sqrt{x_{n+1}} \ge \sqrt{x_1 + x_2 +...+ x_n + x_{n+1}}$$ 
From the hypothesis we have:
$$\sqrt{x_1} + \sqrt{x_2} + ... + \sqrt{x_n} + \sqrt{x_{n+1}} \ge \sqrt{x_1 + x_2 + ... + x_n} + \sqrt{x_{n + 1}} \ge \sqrt{x_1 + x_2 +...+ x_n + x_{n+1}}$$ 
Squaring both sides of the right part:
$$ x_1 + x_2 + ... + x_n + x_{n + 1} + 2\sqrt{x_{n+1}(x_1 + x_2 +... + x_n)} \ge x_1 + x_2 +...+ x_n + x_{n+1} $$
Which is true, hence $P(n + 1)$ is true as well.
I'm not sure if I did it correctly?
 [/INST] I'm will just show it my way, since I find it simpler.
First, we will start off with the basis step where $n = 1$.
Basis step
$\sqrt{x_1} \geq \sqrt{x_1}$, which is of course true.
Inductive step
We let the inequality be true for any $k$, where $0 \leq k \leq n$:
$\sqrt{x_1} + \sqrt{x_2} + ... + \sqrt{x_k} \geq \sqrt{x_1 + x_2 + ... + x_k}$
If this inequality must hold, the inequalty must, by induction, hold for $k + 1$:
$\sqrt{x_1} + \sqrt{x_2} + ... + \sqrt{x_k} + \sqrt{x_{k + 1}} \geq \sqrt{x_1 + x_2 + ... + x_k + x_{k + 1}}$
We add $x_{k + 1}$ to both sides of the inequality:
$\sqrt{x_1} + \sqrt{x_2} + ... + \sqrt{x_k} + \sqrt{x_{k + 1}} \geq \sqrt{x_1 + x_2 + ... + x_k} + \sqrt{x_{k + 1}} \geq \sqrt{x_1 + x_2 + ... + x_k + x_{k + 1}}$
The inequality is hereby proved.
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