<s>[INST] For $a,b,c \in \mathbb{R}$ and $|a| \geq |b+c|, |b| \geq |c+a|, |c| \geq |a+b|$ prove then $a+b+c = 0$ 
For $a,b,c \in \mathbb{R}$ and $|a| \geq |b+c|, |b| \geq |c+a|, |c|
 \geq |a+b|$ prove then $a+b+c = 0$


Solution:
$$
|a| + |b| + |c| \geq |b+c| + |c+a| + |a+b| \geq 0
$$
and
$$0 \leq |a+b+c|\leq |a+b| + |c| \leq |a| + |b| + |c|$$
so we have 
$$0 \leq |a+b+c| \geq 0 $$
$$ |a+b+c|= 0 \iff a+b+c =0 $$ 
I need check my solution. 
Many thanks!
 [/INST] We have:
$$a^2\geq(b+c)^2,$$
$$b^2\geq(a+c)^2$$ and $$c^2\geq(a+b)^2$$ or
$$(a-b-c)(a+b+c)\geq0,$$ $$(b-a-c)(a+b+c)\geq0$$ and $$(c-a-b)(a+b+c)\geq0.$$
Now, for $a+b+c>0$ we obtain:
$$a-b-c\geq0,$$ $$b-a-c\geq0$$ and $$c-a-b\geq0,$$ which gives
$$a-b-c+b-a-c\geq0$$ or
$$c\leq0.$$
Similarly, $b\leq0$ and $a\leq0,$ which gives
$$a+b+c\leq0,$$ which is a contradiction.
By the same way we'll obtain a contradiction for $a+b+c<0.$
Thus, $a+b+c=0$ and we are done because for $a=b=c=0$ we obtain $a+b+c=0.$
</s>