# Usage of triangle inequality?

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• MiddleEast
In summary: It is also used in optimization problems in Calculus and Linear Algebra. There are many other inequalities used in mathematics such as the Cauchy-Schwarz inequality and the AM-GM inequality, but the triangle inequality is a fundamental and widely applicable one.

#### MiddleEast

Hi,
Recently I studied triangle inequality and the proof using textbook precalculus by David Cohen.
My question is whats the benefit of this inequality ? One benefit I found is to solve inequality of the form |x+a| + |x+b| < c which make the solution much easier than taking cases. I assume this inequality can be used in proof? the beauty of this inequality is to separate absolute of sum to sum of absolutes which - supposedly - will make proving (whatever the proof is) much easier.

Are there any other benefits ?
Are there any important inequality other triangle and AM-GM inequality that quite famous ?
Thanks.

The triangle inequality is a fundamental defining property of a distance function or metric (of which ##| x - y|## is probably the first you'll encounter). If you have a set and you want to have a notion of the distance between two elements of that set, which we'll denote by ##d(x, y)##, then we have four fundamental properties. Here ##x, y, z## are any elements in your set.
$$\text{1)} \ d(x, y) \ge 0$$$$\text{2)} \ d(x, y) = 0 \ \Leftrightarrow \ x = y$$$$\text{3)} \ d(x, y) = d(y, x)$$$$\text{4)} \ \text{(the triangle inequality)} \ d(x, z) \le d(x, y) + d(y, z)$$
In any case, the triangle inequality is used all over mathematics and physics.

jbriggs444
MiddleEast said:
Hi,
Recently I studied triangle inequality and the proof using textbook precalculus by David Cohen.
My question is whats the benefit of this inequality ? One benefit I found is to solve inequality of the form |x+a| + |x+b| < c which make the solution much easier than taking cases. I assume this inequality can be used in proof? the beauty of this inequality is to separate absolute of sum to sum of absolutes which - supposedly - will make proving (whatever the proof is) much easier.

Are there any other benefits ?
Are there any important inequality other triangle and AM-GM inequality that quite famous ?
Thanks.
As Perok mentioned, thats the idea of the triangle inequality. It is also a useful tool for proving properties of limits of sequences and functions in Analysis, Topologies with a metric...