Triangle inequality

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In geometry , the triangle inequality is a theorem that says that one side of a triangle is at most as long as the sum of the other two sides. The “at most” includes the special case of equality. The triangle inequality also plays an important role in other areas of mathematics such as linear algebra or functional analysis .

Shapes of the triangle inequality

Triangle inequality for triangles


After the triangle is in the triangle , the sum of the lengths of two sides and always at least as great as the length of the third side . This means formally:

One can also say that the distance from A to B is always at most as large as the distance from A to C and from C to B combined, or to put it in a popular way: "The direct route is always the shortest."

The equals sign only applies if and sections are from - one also speaks of the triangle being "degenerate".

Since, for reasons of symmetry, the following also applies, it follows , analogously, that the total is


The left inequality is also sometimes referred to as the reverse triangle inequality .

The triangle inequality characterizes distance and magnitude functions . It is therefore used as an axiom of the abstract distance function in metric spaces .

Triangle inequality for real numbers

For real numbers the following applies:


Because both sides of the inequality are not negative, squaring is an equivalence transformation :
By deleting identical terms, we arrive at the equivalent inequality
This inequality holds because for any

Inverse triangle inequality

As with the triangle, an inverse triangle inequality can be derived:

There is a substitution of there

if you set it instead, it results

so together (because for any real numbers and with and also applies )

If you replace with so you also get

So overall

for all

Triangle inequality for complex numbers

The following applies to complex numbers :


Since all sides are nonnegative, squaring is an equivalence transformation and one obtains
where the overline means complex conjugation . If you delete identical terms and set so remains
to show. With you get
which is always fulfilled because of and the monotony of the (real) root function.

As in the real case, this inequality also follows

for all

Triangle inequality of absolute value functions for bodies

Along with other demands, an amount function for a body is also given by the

Triangle inequality

established. It has to apply to all If all requirements (see article amount function ) are fulfilled, then an amount function is for the body

Is for all whole , then the amount is called non-Archimedean , otherwise Archimedean .

For non-Archimedean amounts, the

tightened triangle inequality

It makes the amount an ultrametric . Conversely, any ultrametric amount is non-Archimedean.

Triangle inequality for sums and integrals

Repeated application of the triangle inequality or complete induction results

for real or complex numbers . This inequality also applies when integrals are considered instead of sums:

If , where is an interval, Riemann integrable , then it holds


This also applies to complex-valued functions , cf. Then there is a complex number such that

and .


is real, must be zero. Also applies


so overall


Triangle inequality for vectors

The following applies to vectors :


The validity of this relationship can be seen by squaring it


using the Cauchy-Schwarz inequality :


Here, too, it follows as in the real case

such as

Triangle inequality for spherical triangles

Two spherical triangles

The triangle inequality generally does not hold in spherical triangles .

However, it applies if you limit yourself to Eulerian triangles, i.e. those in which each side is shorter than half a great circle.

In the adjacent figure, the following applies

however is .

Triangle inequality for normalized spaces

In a normalized space , the triangle inequality becomes in the form

required as one of the properties that the standard must meet for everyone . In particular, it also follows here

such as

for everyone .

In the special case of L p -spaces , the triangle inequality is called the Minkowski inequality and is proven by means of Hölder's inequality .

Triangle inequality for metric spaces

In a metric space , an axiom for the abstract distance function is required that the triangle inequality in the form

is fulfilled for all . In every metric space, the triangle inequality applies by definition. From this it can be deduced that in a metric space also the reverse triangle inequality

applies to all . In addition, the inequality holds for any


See also

Individual evidence

  1. Harro Heuser: Textbook of Analysis, Part 1. 8th edition. BG Teubner, Stuttgart 1990, ISBN 3-519-12231-6 . Theorem 85.1
  2. ^ Walter Rudin: Real and Complex Analysis . MacGraw-Hill, 1986, ISBN 0-07-100276-6 . Theorem 1.33