Equilateral triangle

Summary

In geometry, an equilateral triangle is a triangle in which all three sides have the same length. In the familiar Euclidean geometry, an equilateral triangle is also equiangular; that is, all three internal angles are also congruent to each other and are each 60°. It is also a regular polygon, so it is also referred to as a regular triangle.

Equilateral triangle
TypeRegular polygon
Edges and vertices3
Schläfli symbol{3}
Coxeter–Dynkin diagrams
Symmetry groupD3
Area
Internal angle (degrees)60°

Principal properties

edit
 
An equilateral triangle. It has equal sides ( ), equal angles ( ), and equal altitudes ( ).

Denoting the common length of the sides of the equilateral triangle as  , we can determine using the Pythagorean theorem that:

  • The area is  
  • The perimeter is  
  • The radius of the circumscribed circle is  
  • The radius of the inscribed circle is   or  
  • The geometric center of the triangle is the center of the circumscribed and inscribed circles
  • The altitude (height) from any side is  

Denoting the radius of the circumscribed circle as R, we can determine using trigonometry that:

  • The area of the triangle is  

Many of these quantities have simple relationships to the altitude ("h") of each vertex from the opposite side:

  • The area is  
  • The height of the center from each side, or apothem, is  
  • The radius of the circle circumscribing the three vertices is  
  • The radius of the inscribed circle is  

In an equilateral triangle, the altitudes, the angle bisectors, the perpendicular bisectors, and the medians to each side coincide.

Characterizations

edit

A triangle   that has the sides  ,  ,  , semiperimeter  , area  , exradii  ,  ,   (tangent to  ,  ,   respectively), and where   and   are the radii of the circumcircle and incircle respectively, is equilateral if and only if any one of the statements in the following nine categories is true. Thus these are properties that are unique to equilateral triangles, and knowing that any one of them is true directly implies that we have an equilateral triangle.

Sides

edit
  •  
  •  [1]

Semiperimeter

edit
  •  [2] (Blundon)
  •  [3]
  •  [4]
  •  
  •  

Angles

edit
  •  
  •  
  •  [5]

Area

edit
  •   (Weitzenböck)
  •  [4]

Circumradius, inradius, and exradii

edit
  •  [6] (Chapple-Euler)
  •  [6]
  •  [5]
  •  

Equal cevians

edit

Three kinds of cevians coincide, and are equal, for (and only for) equilateral triangles:[7]

Coincident triangle centers

edit

Every triangle center of an equilateral triangle coincides with its centroid, which implies that the equilateral triangle is the only triangle with no Euler line connecting some of the centers. For some pairs of triangle centers, the fact that they coincide is enough to ensure that the triangle is equilateral. In particular:

Six triangles formed by partitioning by the medians

edit

For any triangle, the three medians partition the triangle into six smaller triangles.

  • A triangle is equilateral if and only if any three of the smaller triangles have either the same perimeter or the same inradius.[9]: Theorem 1 
  • A triangle is equilateral if and only if the circumcenters of any three of the smaller triangles have the same distance from the centroid.[9]: Corollary 7 

Points in the plane

edit
  • A triangle is equilateral if and only if, for every point   in the plane, with distances  ,  , and   to the triangle's sides and distances  ,  , and   to its vertices,[10]: p.178, #235.4 
     

Notable theorems

edit
 
Visual proof of Viviani's theorem
  1. Nearest distances from point P to sides of equilateral triangle   are shown.
  2. Lines  ,  , and   parallel to  ,   and  , respectively, define smaller triangles  ,   and  .
  3. As these triangles are equilateral, their altitudes can be rotated to be vertical.
  4. As   is a parallelogram, triangle   can be slid up to show that the altitudes sum to that of triangle  .

Morley's trisector theorem states that, in any triangle, the three points of intersection of the adjacent angle trisectors form an equilateral triangle.

Napoleon's theorem states that, if equilateral triangles are constructed on the sides of any triangle, either all outward, or all inward, the centers of those equilateral triangles themselves form an equilateral triangle.

A version of the isoperimetric inequality for triangles states that the triangle of greatest area among all those with a given perimeter is equilateral.[11]

Viviani's theorem states that, for any interior point   in an equilateral triangle with distances  ,  , and   from the sides and altitude  ,

 
independent of the location of  .[12]

Pompeiu's theorem states that, if   is an arbitrary point in the plane of an equilateral triangle   but not on its circumcircle, then there exists a triangle with sides of lengths  ,  , and  . That is,  ,  , and   satisfy the triangle inequality that the sum of any two of them is greater than the third. If   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.

Geometric construction

edit
 
Construction of equilateral triangle with compass and straightedge

An equilateral triangle is easily constructed using a straightedge and compass, because 3 is a Fermat prime. Draw a straight line, and place the point of the compass on one end of the line, and swing an arc from that point to the other point of the line segment. Repeat with the other side of the line. Finally, connect the point where the two arcs intersect with each end of the line segment.

An alternative method is to draw a circle with radius  , place the point of the compass on the circle and draw another circle with the same radius. The two circles will intersect in two points. An equilateral triangle can be constructed by taking the two centers of the circles and either of the points of intersection.

In both methods a by-product is the formation of vesica piscis.

The proof that the resulting figure is an equilateral triangle is the first proposition in Book I of Euclid's Elements.

 

Derivation of area formula

edit

The area formula   in terms of side length   can be derived directly using the Pythagorean theorem or using trigonometry.

Using the Pythagorean theorem

edit

The area of a triangle is half of one side   times the height   from that side:

 
 
An equilateral triangle with a side of 2 has a height of 3, as the sine of 60° is 3/2.

The legs of either right triangle formed by an altitude of the equilateral triangle are half of the base  , and the hypotenuse is the side   of the equilateral triangle. The height of an equilateral triangle can be found using the Pythagorean theorem

 
so that
 

Substituting   into the area formula   gives the area formula for the equilateral triangle:

 

Using trigonometry

edit

Using trigonometry, the area of a triangle with any two sides   and  , and an angle   between them is

 

Each angle of an equilateral triangle is 60°, so

 

The sine of 60° is  . Thus

 
since all sides of an equilateral triangle are equal.

Other properties

edit

An equilateral triangle is the most symmetrical triangle, having 3 lines of reflection and rotational symmetry of order 3 about its center, whose symmetry group is the dihedral group of order 6,  . The integer-sided equilateral triangle is the only triangle with integer sides, and three rational angles as measured in degrees.[13] It is the only acute triangle that is similar to its orthic triangle (with vertices at the feet of the altitudes),[14]: p. 19  and the only triangle whose Steiner inellipse is a circle (specifically, the incircle). The triangle of largest area of all those inscribed in a given circle is equilateral, and the triangle of smallest area of all those circumscribed around a given circle is also equilateral.[15] It is the only regular polygon aside from the square that can be inscribed inside any other regular polygon.

By Euler's inequality, the equilateral triangle has the smallest ratio of the circumradius   to the inradius   of any triangle, with[16]: p.198 

 

Given a point   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   is the centroid. In no other triangle is there a point for which this ratio is as small as 2.[17] 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   to the points where the angle bisectors of  ,  , and   cross the sides ( ,  , and   being the vertices). There are numerous other triangle inequalities that hold with equality if and only if the triangle is equilateral.

For any point   in the plane, with distances  ,  , and   from the vertices  ,  , and   respectively,[18]

 

For any point   in the plane, with distances  ,  , and   from the vertices,[19]

 
 
where   is the circumscribed radius and   is the distance between point   and the centroid of the equilateral triangle.

For any point   on the inscribed circle of an equilateral triangle, with distances  ,  , and   from the vertices,[20]

 
 

For any point   on the minor arc   of the circumcircle, with distances  ,  , and   from  ,  , and  , respectively[12]

 
 

Moreover, if point   on side   divides   into segments   and   with   having length   and   having length  , then[12]: 172 

 
which also equals   if   and
 
which is the optic equation.

For an equilateral triangle:

  • The ratio of its area to the area of the incircle,  , is the largest of any triangle.[21]: Theorem 4.1 
  • The ratio of its area to the square of its perimeter,   is larger than that of any non-equilateral triangle.[11]
  • If a segment splits an equilateral triangle into two regions with equal perimeters and with areas   and  , then[10]: p.151, #J26 

 

If a triangle is placed in the complex plane with complex vertices  ,  , and  , then for either non-real cube root   of 1 the triangle is equilateral if and only if[22]: Lemma 2 

 
 
The equilateral triangle tiling fills the plane.

Notably, the equilateral triangle tiles two dimensional space with six triangles meeting at a vertex, whose dual tessellation is the hexagonal tiling. 3.122, 3.4.6.4, (3.6)2, 32.4.3.4, and 34.6 are all semi-regular tessellations constructed with equilateral triangles.[23]

 
A regular tetrahedron is made of four equilateral triangles.

In three dimensions, equilateral triangles form faces of regular and uniform polyhedra. Three of the five Platonic solids are composed of equilateral triangles: the tetrahedron, octahedron and icosahedron.[24]: p.238  In particular, the tetrahedron, which has four equilateral triangles for faces, can be considered the three-dimensional analogue of the triangle. All Platonic solids can inscribe tetrahedra, as well as be inscribed inside tetrahedra. Equilateral triangles also form uniform antiprisms as well as uniform star antiprisms in three-dimensional space. For antiprisms, two (non-mirrored) parallel copies of regular polygons are connected by alternating bands of   equilateral triangles.[25] Specifically for star antiprisms, there are prograde and retrograde (crossed) solutions that join mirrored and non-mirrored parallel star polygons.[26][27] The Platonic octahedron is also a triangular antiprism, which is the first true member of the infinite family of antiprisms (the tetrahedron, as a digonal antiprism, is sometimes considered the first).[24]: p.240 

As a generalization, the equilateral triangle belongs to the infinite family of  -simplexes, with  .[28]

In culture and society

edit

Equilateral triangles have frequently appeared in man made constructions:

See also

edit

References

edit
  1. ^ Bencze, Mihály; Wu, Hui-Hua; Wu, Shan-He (2008). "An equivalent form of fundamental triangle inequality and its applications" (PDF). Journal of Inequalities in Pure and Applied Mathematics. 10 (1): 1–6 (Article No. 16). ISSN 1443-5756. MR 2491926. S2CID 115305257. Zbl 1163.26316.
  2. ^ Dospinescu, G.; Lascu, M.; Pohoata, C.; Letiva, M. (2008). "An elementary proof of Blundon's inequality" (PDF). Journal of Inequalities in Pure and Applied Mathematics. 9 (4): 1-3 (Paper No. 100). ISSN 1443-5756. S2CID 123965364. Zbl 1162.51305.
  3. ^ Blundon, W. J. (1963). "On Certain Polynomials Associated with the Triangle". Mathematics Magazine. 36 (4). Taylor & Francis: 247–248. doi:10.2307/2687913. JSTOR 2687913. S2CID 124726536. Zbl 0116.12902.
  4. ^ a b Alsina, Claudi; Nelsen, Roger B. (2009). When less is more. Visualizing basic inequalities. Dolciani Mathematical Expositions. Vol. 36. Washington, D.C.: Mathematical Association of America. pp. 71, 155. doi:10.5948/upo9781614442028. ISBN 978-0-88385-342-9. MR 2498836. OCLC 775429168. S2CID 117769827. Zbl 1163.00008.
  5. ^ a b Pohoata, Cosmin (2010). "A new proof of Euler's inradius - circumradius inequality" (PDF). Gazeta Matematica Seria B (3): 121–123. S2CID 124244932.
  6. ^ a b c Andreescu, Titu; Andrica, Dorian (2006). Complex Numbers from A to...Z (1st ed.). Boston, MA: Birkhäuser. pp. 70, 113–115. doi:10.1007/0-8176-4449-0. ISBN 978-0-8176-4449-9. OCLC 871539199. S2CID 118951675.
  7. ^ Owen, Byer; Felix, Lazebnik; Deirdre, Smeltzer (2010). Methods for Euclidean Geometry. Classroom Resource Materials. Vol. 37. Washington, D.C.: Mathematical Association of America. pp. 36, 39. doi:10.5860/choice.48-3331. ISBN 9780883857632. OCLC 501976971. S2CID 118179744.
  8. ^ Yiu, Paul (1998). "Notes on Euclidean Geometry" (PDF). Florida Atlantic University, Department of Mathematical Sciences (Course Notes).
  9. ^ a b Cerin, Zvonko (2004). "The vertex-midpoint-centroid triangles" (PDF). Forum Geometricorum. 4: 97–109.
  10. ^ a b "Inequalities proposed in "Crux Mathematicorum"" (PDF).
  11. ^ a b Chakerian, G. D. "A Distorted View of Geometry." Ch. 7 in Mathematical Plums (R. Honsberger, editor). Washington, DC: Mathematical Association of America, 1979: 147.
  12. ^ a b c Posamentier, Alfred S.; Salkind, Charles T. (1996). Challenging Problems in Geometry. Dover Publ.
  13. ^ Conway, J. H., and Guy, R. K., "The only rational triangle", in The Book of Numbers, 1996, Springer-Verlag, pp. 201 and 228–239.
  14. ^ Leon Bankoff and Jack Garfunkel, "The heptagonal triangle", Mathematics Magazine 46 (1), January 1973, 7–19,
  15. ^ Dörrie, Heinrich (1965). 100 Great Problems of Elementary Mathematics. Dover Publ. pp. 379–380.
  16. ^ Svrtan, Dragutin; Veljan, Darko (2012). "Non-Euclidean versions of some classical triangle inequalities" (PDF). Forum Geometricorum. 12: 197–209.
  17. ^ Lee, Hojoo (2001). "Another proof of the Erdős–Mordell Theorem" (PDF). Forum Geometricorum. 1: 7–8.
  18. ^ Gardner, Martin, "Elegant Triangles", in the book Mathematical Circus, 1979, p. 65.
  19. ^ Meskhishvili, Mamuka (2021). "Cyclic Averages of Regular Polygonal Distances" (PDF). International Journal of Geometry. 10: 58–65.
  20. ^ De, Prithwijit (2008). "Curious properties of the circumcircle and incircle of an equilateral triangle" (PDF). Mathematical Spectrum. 41 (1): 32–35.
  21. ^ Minda, D.; Phelps, S. (2008). "Triangles, ellipses, and cubic polynomials". American Mathematical Monthly. 115 (October): 679–689. doi:10.1080/00029890.2008.11920581. JSTOR 27642581. S2CID 15049234.
  22. ^ Dao, Thanh Oai (2015). "Equilateral triangles and Kiepert perspectors in complex numbers" (PDF). Forum Geometricorum. 15: 105–114.
  23. ^ Grünbaum, Branko; Shepard, Geoffrey (November 1977). "Tilings by Regular Polygons" (PDF). Mathematics Magazine. 50 (5). Taylor & Francis, Ltd.: 231–234. doi:10.2307/2689529. JSTOR 2689529. MR 1567647. S2CID 123776612. Zbl 0385.51006.
  24. ^ a b Johnson, Norman W. (2018). Geometries and Transformations (1st ed.). Cambridge: Cambridge University Press. pp. xv, 1–438. doi:10.1017/9781316216477. ISBN 978-1107103405. S2CID 125948074. Zbl 1396.51001.
  25. ^ Cromwell, Peter T. (1997). "Chapter 2: The Archimedean solids". Polyhedra (1st ed.). New York: Cambridge University Press. p. 85. ISBN 978-0521664059. MR 1458063. OCLC 41212721. Zbl 0888.52012.
  26. ^ Klitzing, Richard. "n-antiprism with winding number d". Polytopes & their Incidence Matrices. bendwavy.org (Anton Sherwood). Retrieved 2023-03-09.
  27. ^ Webb, Robert. "Stella Polyhedral Glossary". Stella. Retrieved 2023-03-09.
  28. ^ H. S. M. Coxeter (1948). Regular Polytopes (1 ed.). London: Methuen & Co. LTD. pp. 120–121. OCLC 4766401. Zbl 0031.06502.
  29. ^ Pelkonen, Eeva-Liisa; Albrecht, Donald, eds. (2006). Eero Saarinen: Shaping the Future. Yale University Press. pp. 160, 224, 226. ISBN 978-0972488129.
  30. ^ White, Steven F.; Calderón, Esthela (2008). Culture and Customs of Nicaragua. Greenwood Press. p. 3. ISBN 978-0313339943.
  31. ^ Guillermo, Artemio R. (2012). Historical Dictionary of the Philippines. Scarecrow Press. p. 161. ISBN 978-0810872462.
  32. ^ Riley, Michael W.; Cochran, David J.; Ballard, John L. (December 1982). "An Investigation of Preferred Shapes for Warning Labels". Human Factors: The Journal of the Human Factors and Ergonomics Society. 24 (6): 737–742. doi:10.1177/001872088202400610. S2CID 109362577.
edit
Family An Bn I2(p) / Dn E6 / E7 / E8 / F4 / G2 Hn
Regular polygon Triangle Square p-gon Hexagon Pentagon
Uniform polyhedron Tetrahedron OctahedronCube Demicube DodecahedronIcosahedron
Uniform polychoron Pentachoron 16-cellTesseract Demitesseract 24-cell 120-cell600-cell
Uniform 5-polytope 5-simplex 5-orthoplex5-cube 5-demicube
Uniform 6-polytope 6-simplex 6-orthoplex6-cube 6-demicube 122221
Uniform 7-polytope 7-simplex 7-orthoplex7-cube 7-demicube 132231321
Uniform 8-polytope 8-simplex 8-orthoplex8-cube 8-demicube 142241421
Uniform 9-polytope 9-simplex 9-orthoplex9-cube 9-demicube
Uniform 10-polytope 10-simplex 10-orthoplex10-cube 10-demicube
Uniform n-polytope n-simplex n-orthoplexn-cube n-demicube 1k22k1k21 n-pentagonal polytope
Topics: Polytope familiesRegular polytopeList of regular polytopes and compounds