Understand the problem

Determine the minimal and maximal values the expression $\frac{|a+b|+|b+c|+|c+a|}{|a|+|b|+|c|}$ can take, where $a,b,c$ are real numbers.
Source of the problem
Israel MO 2018, Problem 3
Topic
Algebra, Inequality
Difficulty Level
6/10
Suggested Book
Excursion in Mathematics by Bhaskarcharya Prathisthan

Start with hints

Do you really need a hint? Try it first!

Given the expressions, what inequality comes to your mind first? The triangle inequality right? |x| + |y| \( \geq \) |x+y|. Can you use this inequality to get a maximum bound?
Maximum Bound: Observe that the maximum bound is got by the triangle inequality as explained. $|a+b| \le |a|+|b|$
$|b+c| \le |b|+|c|$
$|a+c| \le |a|+|c|$ We get, $\frac{|a+b|+|b+c|+|c+a|}{|a|+|b|+|c|} \le\frac{|a|+|b|+|b|+|c|+|a|+|c|}{|a|+|b|+|c|}=2$ Never forget to mention the equality case: a = b = c is the equality case.  
What about the minimum inequality? The idea is that you can observe that keeping the denominator constant, can you reduce the numerator. Let’s take the case of the |a| = |b| = |c| = 1. Now, the expression is maximized when a = b = c = 1 or -1. So, obviously one must be positive or two must be negative or vice-versa. In either case, we get \( \frac{2}{3} \). Okay, then maybe we need to deal with the signs and stuff to get a hold on the minimum. Let’s fix the signs of a,b,c then, we can break the bonds of the modulus. Let’s proceed to the next hint.

Let $a$, $b$, and $c$ be arbitrary real numbers, not all of them equal $0$. By flipping signs, we can assume that at least two of $a$, $b$, and $c$ are non-negative. Actually, without loss of generality, we can assume that $a, b\geq 0$. \( 3|a + b| + 3|b + c| + 3|c + a| \geq 3a + 3b + 3|b+c| + 3|a+c| \geq 2(a+b) + (a + |a + c|) + (b + |b + c|) \) \( = 2(|a| + |b|) + (|-a| + |a + c|) + (|-b| + |b + c|) \geq 2(|a| + |b|) + |c| + |c| = 2|a| + 2|b| + 2|c| \) We have proved that the minimum possible value of $\frac{|a+b|+|b+c|+|c+a|}{|a|+|b|+|c|}$ is $\frac{2}{3}$. The minimum is $\frac{2}{3}$, which is attained for $a = b = 1$, $c = -1$.  

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