Triangle inequality

In mathematics, the triangle inequality states that for any triangle, the sum of the lengths of any two sides must be greater than or equal to the length of the remaining side. This statement permits the inclusion of degenerate triangles, but some authors, especially those writing about elementary geometry, will exclude this possibility, thus leaving out the possibility of equality. If,, and are the lengths of the sides of a triangle then the triangle inequality states that
with equality only in the degenerate case of a triangle with zero area.
In Euclidean geometry and some other geometries, the triangle inequality is a theorem about vectors and vector lengths :
where the length of the third side has been replaced by the length of the vector sum. When and are real numbers, they can be viewed as vectors in, and the triangle inequality expresses a relationship between absolute values.
In Euclidean geometry, for right triangles the triangle inequality is a consequence of the Pythagorean theorem, and for general triangles, a consequence of the law of cosines, although it may be proved without these theorems. The inequality can be viewed intuitively in either or. The figure at the right shows three examples beginning with clear inequality and approaching equality. In the Euclidean case, equality occurs only if the triangle has a angle and two angles, making the three vertices collinear, as shown in the bottom example. Thus, in Euclidean geometry, the shortest distance between two points is a straight line.
In spherical geometry, the shortest distance between two points is an arc of a great circle, but the triangle inequality holds provided the restriction is made that the distance between two points on a sphere is the length of a minor spherical line segment with those endpoints.
The triangle inequality is a defining property of norms and measures of distance. This property must be established as a theorem for any function proposed for such purposes for each particular space: for example, spaces such as the real numbers, Euclidean spaces, the L^p spaces, and inner product spaces.

Euclidean geometry

The triangle inequality theorem is stated in Euclid's Elements, Book I, Proposition 20:
in the triangle ABC the sum of any two sides is greater than the remaining one, that is, the sum of BA and AC is greater than BC, the sum of AB and BC is greater than AC, and the sum of BC and CA is greater than AB.
Euclid proved the triangle inequality for distances in plane geometry using the construction in the figure. Beginning with triangle, an isosceles triangle is constructed with one side taken as and the other equal leg along the extension of side. It then is argued that angle has larger measure than angle, so side is longer than side. However:
so the sum of the lengths of sides and is larger than the length of. This proof appears in Euclid's Elements, Book I, Proposition 20.

Mathematical expression of the constraint on the sides of a triangle

For a proper triangle, the triangle inequality, as stated in words, literally translates into three inequalities :
A more succinct form of this inequality system can be shown to be
Another way to state it is
implying
and thus that the longest side length is less than the semiperimeter.
A mathematically equivalent formulation is that the area of a triangle with sides must be a real number greater than zero. Heron's formula for the area is
In terms of either area expression, the triangle inequality imposed on all sides is equivalent to the condition that the expression under the square root sign be real and greater than zero.
The triangle inequality provides two more interesting constraints for triangles whose sides are, where and is the golden ratio, as

Right triangle

In the case of right triangles, the triangle inequality specializes to the statement that the hypotenuse is greater than either of the two sides and less than their sum.
The second part of this theorem is already established above for any side of any triangle. The first part is established using the lower figure. In the figure, consider the right triangle. An isosceles triangle is constructed with equal sides. From the triangle postulate, the angles in the right triangle satisfy:
Likewise, in the isosceles triangle, the angles satisfy:
Therefore,
and so, in particular,
That means side, which is opposite to angle, is shorter than side, which is opposite to the larger angle. But. Hence:
A similar construction shows, establishing the theorem.
An alternative proof proceeds by considering three positions for point : as depicted, or coincident with , or lastly, interior to the right triangle between points and .
This theorem establishing inequalities is sharpened by Pythagoras' theorem to the equality that the square of the length of the hypotenuse equals the sum of the squares of the other two sides.

Examples of use

Consider a triangle whose sides are in an arithmetic progression and let the sides be. Then the triangle inequality requires that
To satisfy all these inequalities requires
When is chosen such that, it generates a right triangle that is always similar to the Pythagorean triple with sides,,.
Now consider a triangle whose sides are in a geometric progression and let the sides be. Then the triangle inequality requires that
The first inequality requires ; consequently it can be divided through and eliminated. With, the middle inequality only requires. This now leaves the first and third inequalities needing to satisfy
The first of these quadratic inequalities requires to range in the region beyond the value of the positive root of the quadratic equation, i.e. where is the golden ratio. The second quadratic inequality requires to range between 0 and the positive root of the quadratic equation, i.e.. The combined requirements result in being confined to the range
When the common ratio is chosen such that it generates a right triangle that is always similar to the Kepler triangle.

Generalization to any polygon

The triangle inequality can be extended by mathematical induction to arbitrary polygonal paths, showing that the total length of such a path is no less than the length of the straight line between its endpoints. Consequently, the length of any polygon side is always less than the sum of the other polygon side lengths.

Example of the generalized polygon inequality for a quadrilateral

Consider a quadrilateral whose sides are in a geometric progression and let the sides be. Then the generalized polygon inequality requires that
These inequalities for reduce to the following
The left-hand side polynomials of these two inequalities have roots that are the tribonacci constant and its reciprocal. Consequently, is limited to the range where is the tribonacci constant.

Relationship with shortest paths

This generalization can be used to prove that the shortest curve between two points in Euclidean geometry is a straight line.
No polygonal path between two points is shorter than the line between them. This implies that no curve can have an arc length less than the distance between its endpoints. By definition, the arc length of a curve is the least upper bound of the lengths of all polygonal approximations of the curve. The result for polygonal paths shows that the straight line between the endpoints is the shortest of all the polygonal approximations. Because the arc length of the curve is greater than or equal to the length of every polygonal approximation, the curve itself cannot be shorter than the straight line path.

Converse

The converse of the triangle inequality theorem is also true: if three real numbers are such that each is less than the sum of the others, then there exists a triangle with these numbers as its side lengths and with positive area; and if one number equals the sum of the other two, there exists a degenerate triangle with these numbers as its side lengths.
In either case, if the side lengths are,, we can attempt to place a triangle in the Euclidean plane as shown in the diagram. We need to prove that there exists a real number consistent with the values,, and, in which case this triangle exists.
Image:Triangle with notations 3.svg|thumb|Triangle with altitude cutting base into.
By the Pythagorean theorem we have and according to the figure at the right. Subtracting these yields. This equation allows us to express in terms of the sides of the triangle:
For the height of the triangle we have that. By replacing with the formula given above, we have
For a real number to satisfy this, must be non-negative:
which holds if the triangle inequality is satisfied for all sides. Therefore, there does exist a real number consistent with the sides, and the triangle exists. If each triangle inequality holds strictly, and the triangle is non-degenerate ; but if one of the inequalities holds with equality, so, the triangle is degenerate.

Generalization to higher dimensions

The area of a triangular face of a tetrahedron is less than or equal to the sum of the areas of the other three triangular faces. More generally, in Euclidean space the hypervolume of an -facet of an -simplex is less than or equal to the sum of the hypervolumes of the other facets.
Much as the triangle inequality generalizes to a polygon inequality, the inequality for a simplex of any dimension generalizes to a polytope of any dimension: the hypervolume of any facet of a polytope is less than or equal to the sum of the hypervolumes of the remaining facets.
In some cases the tetrahedral inequality is stronger than several applications of the triangle inequality. For example, the triangle inequality appears to allow the possibility of four points,,, and in Euclidean space such that distances
and
However, points with such distances cannot exist: the area of the equilateral triangle is, which is larger than three times, the area of a isosceles triangle, and so the arrangement is forbidden by the tetrahedral inequality.