Editing Equilateral triangle (section)

== Properties ==
An equilateral triangle is a triangle that has three equal sides. It is a special case of an [[isosceles triangle]] in the modern definition, stating that an isosceles triangle is defined at least as having two equal sides.{{sfnp|Stahl|2003|p=[https://books.google.com/books?id=jLk7lu3bA1wC&pg=PA37 37]}} Based on the modern definition, this leads to an equilateral triangle in which one of the three sides may be considered its base.{{sfnp|Lardner|1840|p=46}}

The follow-up definition above may result in more precise properties. For example, since the [[perimeter]] of an isosceles triangle is the sum of its two legs and base, the equilateral triangle is formulated as three times its side.{{sfnp|Harris|Stocker|1998|p=[https://books.google.com/books?id=DnKLkOb_YfIC&pg=PA78 78]}}{{sfnp|Cerin|2004|loc=See Theorem 1}} The [[internal angle]] of an equilateral triangle are equal, 60°.{{sfnp|Owen|Felix|Deirdre|2010|p=36, 39}} Because of these properties, the equilateral triangles are [[regular polygon]]s. The [[cevian]]s of an equilateral triangle are all equal in length, resulting in the [[median]] and [[angle bisector]] being equal in length, considering those lines as their altitude depending on the base's choice.{{sfnp|Owen|Felix|Deirdre|2010|p=36, 39}} When the equilateral triangle is flipped across its altitude or rotated around its center for one-third of a full turn, its appearance is unchanged; it has the symmetry of a [[dihedral group]] <math> \mathrm{D}_3 </math> of [[Dihedral group of order 6|order six]].{{sfnp|Carstensen|Fine|Rosenberger|2011|p=[https://books.google.com/books?id=X1SJ_ywbgy8C&pg=PA156 156]}} Other properties are discussed below.

=== Area ===
[[File:Equilateral triangle with height square root of 3.svg|thumb|The right triangle with a [[hypotenuse]] of <math> 1 </math> has a height of <math> \sqrt{3}/2 </math>, the sine of 60°.]]
The area of an equilateral triangle with edge length <math> a </math> is
<math display="block"> T = \frac{\sqrt{3}}{4}a^2. </math>
The formula may be derived from the formula of an isosceles triangle by [[Pythagoras theorem]]: the altitude <math> h </math> of a triangle is [[Isosceles triangle#Height|the square root of the difference of squares of a side and half of a base]].{{sfnp|Harris|Stocker|1998|p=[https://books.google.com/books?id=DnKLkOb_YfIC&pg=PA78 78]}} Since the base and the legs are equal, the height is:{{sfnp|McMullin|Parkinson|1936|p=[https://books.google.com/books?id=6RA8AAAAIAAJ&pg=PA96 96]}}
<math display="block"> h = \sqrt{a^2 - \frac{a^2}{4}} = \frac{\sqrt{3}}{2}a. </math>
In general, the [[Area_of_a_triangle|area of a triangle]] is half the product of its base and height. The formula for the area of an equilateral triangle can be obtained by substituting the altitude formula.{{sfnp|McMullin|Parkinson|1936|p=[https://books.google.com/books?id=6RA8AAAAIAAJ&pg=PA96 96]}} Another way to prove the area of an equilateral triangle is by using the [[trigonometric function]]. The area of a triangle is formulated as the half product of base and height and the sine of an angle. Because all of the angles of an equilateral triangle are 60°, the formula is as desired.{{citation needed|date=September 2024}}

A version of the [[isoperimetric inequality#Isoperimetric inequality for triangles|isoperimetric inequality for triangles]] states that the triangle of greatest [[area]] among all those with a given [[perimeter]] is equilateral. That is, for perimeter <math> p </math> and area <math> T </math>, the equality holds for the equilateral triangle:{{sfnp|Chakerian|1979}}
<math display="block"> p^2 = 12\sqrt{3}T. </math>

=== Relationship with circles ===
The radius of the [[circumscribed circle]] is:
<math display="block"> R = \frac{a}{\sqrt{3}}, </math>
and the radius of the [[incircle and excircles of a triangle|inscribed circle]] is half of the circumradius:
<math display="block"> r = \frac{\sqrt{3}}{6}a. </math>

A [[Euler's theorem in geometry|theorem of Euler]] states that the distance <math> t </math> between circumcenter and incenter is formulated as <math> t^2 = R(R - 2r) </math>. As a corollary of this, the equilateral triangle has the smallest ratio of the circumradius <math>R</math> to the inradius <math>r</math> of any triangle. That is:{{sfnp|Svrtan|Veljan|2012}}
<math display="block"> R \ge 2r. </math>

[[Pompeiu's theorem]] states that, if <math>P</math> is an arbitrary point in the plane of an equilateral triangle <math>ABC</math> but not on its [[circumcircle]], then there exists a triangle with sides of lengths <math>PA</math>, <math>PB</math>, and <math>PC</math>. That is, <math>PA</math>, <math>PB</math>, and <math>PC</math> satisfy the [[triangle inequality]] that the sum of any two of them is greater than the third. If <math>P</math> is on the circumcircle then the sum of the two smaller ones equals the longest and the triangle has degenerated into a line, this case is known as [[Van Schooten's theorem]].{{sfnp|Alsina|Nelsen|2010|p=[https://books.google.com/books?id=mIT5-BN_L0oC&pg=PA102 102&ndash;103]}}

A [[packing problem]] asks the objective of [[Circle packing in an equilateral triangle|<math> n </math> circles packing into the smallest possible equilateral triangle]]. The optimal solutions show <math> n < 13 </math> that can be packed into the equilateral triangle, but the open conjectures expand to <math> n < 28 </math>.{{sfnp|Melissen|Schuur|1995}}

=== Other mathematical properties ===
[[File:Viviani_theorem_visual_proof.svg|thumb|Visual proof of Viviani's theorem]]
[[Morley's trisector theorem]] states that, in any triangle, the three points of intersection of the adjacent [[angle trisection|angle trisectors]] form an equilateral triangle.

[[Viviani's theorem]] states that, for any interior point <math>P</math> in an equilateral triangle with distances <math>d</math>, <math>e</math>, and <math>f</math> from the sides and altitude <math>h</math>, <math display="block">d+e+f = h,</math> independent of the location of <math>P</math>.{{sfnp|Posamentier|Salkind|1996}}

An equilateral triangle may have [[Integer triangle|integer sides]] with three rational angles as measured in degrees,{{sfnp|Conway|Guy|1996|p=201, 228&ndash;229}} known for the only acute triangle that is similar to its [[orthic triangle]] (with vertices at the feet of the [[altitude (geometry)|altitudes]]),{{sfnp|Bankoff|Garfunkel|1973|p=19}} and the only triangle whose [[Steiner inellipse]] is a circle (specifically, the incircle). The triangle of the largest area of all those inscribed in a given circle is equilateral, and the triangle of the smallest area of all those circumscribed around a given circle is also equilateral.{{sfnp|Dörrie|1965|p=379&ndash;380}} It is the only regular polygon aside from the [[square]] that can be [[inscribed]] inside any other regular polygon.

Given a point <math> P </math> in the interior of an equilateral triangle, the ratio of the sum of its distances from the vertices to the sum of its distances from the sides is greater than or equal to 2, equality holding when <math>P</math> is the centroid. In no other triangle is there a point for which this ratio is as small as 2.{{sfnp|Lee|2001}} This is the [[Erdős–Mordell inequality]]; a stronger variant of it is [[Barrow's inequality]], which replaces the perpendicular distances to the sides with the distances from <math>P</math> to the points where the [[angle bisector]]s of <math>\angle APB</math>, <math>\angle BPC</math>, and <math>\angle CPA</math> cross the sides (<math>A</math>, <math>B</math>, and <math>C</math> being the vertices). There are numerous other [[list of triangle inequalities|triangle inequalities]] that hold equality if and only if the triangle is equilateral.