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In geometry, a square is a regular quadrilateral, which means that it has four equal sides and four equal angles (90-degree angles, or (100-gradian angles or right angles).[1] It can also be defined as a rectangle in which two adjacent sides have equal length. A square with vertices ABCD would be denoted

displaystyle square

ABCD.

Contents

1 Characterizations 2 Properties

2.1 Perimeter
Perimeter
and area 2.2 Other facts

3 Coordinates and equations 4 Construction 5 Symmetry 6 Squares inscribed in triangles 7 Squaring the circle 8 Non-Euclidean geometry 9 Crossed square 10 Graphs 11 See also 12 References 13 External links

Characterizations A convex quadrilateral is a square if and only if it is any one of the following:[2][3]

a rectangle with two adjacent equal sides a rhombus with a right vertex angle a rhombus with all angles equal a parallelogram with one right vertex angle and two adjacent equal sides a quadrilateral with four equal sides and four right angles a quadrilateral where the diagonals are equal and are the perpendicular bisectors of each other, i.e. a rhombus with equal diagonals a convex quadrilateral with successive sides a, b, c, d whose area is

A =

1 2

(

a

2

+

c

2

) =

1 2

(

b

2

+

d

2

) .

displaystyle A= tfrac 1 2 (a^ 2 +c^ 2 )= tfrac 1 2 (b^ 2 +d^ 2 ).

[4]:Corollary 15

Properties A square is a special case of a rhombus (equal sides, opposite equal angles), a kite (two pairs of adjacent equal sides), a trapezoid (one pair of opposite sides parallel), a parallelogram (all opposite sides parallel), a quadrilateral or tetragon (four-sided polygon), and a rectangle (opposite sides equal, right-angles) and therefore has all the properties of all these shapes, namely:[5]

The diagonals of a square bisect each other and meet at 90° The diagonals of a square bisect its angles. Opposite sides of a square are both parallel and equal in length. All four angles of a square are equal. (Each is 360°/4 = 90°, so every angle of a square is a right angle.) All four sides of a square are equal. The diagonals of a square are equal. The square is the n=2 case of the families of n-hypercubes and n-orthoplexes. A square has Schläfli symbol
Schläfli symbol
4 . A truncated square, t 4 , is an octagon, 8 . An alternated square, h 4 , is a digon, 2 .

Perimeter
Perimeter
and area

The area of a square is the product of the length of its sides.

The perimeter of a square whose four sides have length

displaystyle ell

is

P = 4 ℓ

displaystyle P=4ell

and the area A is

A =

2

.

displaystyle A=ell ^ 2 .

In classical times, the second power was described in terms of the area of a square, as in the above formula. This led to the use of the term square to mean raising to the second power. The area can also be calculated using the diagonal d according to

A =

d

2

2

.

displaystyle A= frac d^ 2 2 .

In terms of the circumradius R, the area of a square is

A = 2

R

2

;

displaystyle A=2R^ 2 ;

since the area of the circle is

π

R

2

,

displaystyle pi R^ 2 ,

the square fills approximately 0.6366 of its circumscribed circle. In terms of the inradius r, the area of the square is

A = 4

r

2

.

displaystyle A=4r^ 2 .

Because it is a regular polygon, a square is the quadrilateral of least perimeter enclosing a given area. Dually, a square is the quadrilateral containing the largest area within a given perimeter.[6] Indeed, if A and P are the area and perimeter enclosed by a quadrilateral, then the following isoperimetric inequality holds:

16 A ≤

P

2

displaystyle 16Aleq P^ 2

with equality if and only if the quadrilateral is a square. Other facts

The diagonals of a square are

2

displaystyle scriptstyle sqrt 2

(about 1.414) times the length of a side of the square. This value, known as the square root of 2 or Pythagoras' constant, was the first number proven to be irrational. A square can also be defined as a parallelogram with equal diagonals that bisect the angles. If a figure is both a rectangle (right angles) and a rhombus (equal edge lengths), then it is a square. If a circle is circumscribed around a square, the area of the circle is

π

/

2

displaystyle pi /2

(about 1.5708) times the area of the square. If a circle is inscribed in the square, the area of the circle is

π

/

4

displaystyle pi /4

(about 0.7854) times the area of the square. A square has a larger area than any other quadrilateral with the same perimeter.[7] A square tiling is one of three regular tilings of the plane (the others are the equilateral triangle and the regular hexagon). The square is in two families of polytopes in two dimensions: hypercube and the cross-polytope. The Schläfli symbol
Schläfli symbol
for the square is  4 . The square is a highly symmetric object. There are four lines of reflectional symmetry and it has rotational symmetry of order 4 (through 90°, 180° and 270°). Its symmetry group is the dihedral group D4. If the inscribed circle of a square ABCD has tangency points E on AB, F on BC, G on CD, and H on DA, then for any point P on the inscribed circle,[8]

2 ( P

H

2

− P

E

2

) = P

D

2

− P

B

2

.

displaystyle 2(PH^ 2 -PE^ 2 )=PD^ 2 -PB^ 2 .

If

d

i

displaystyle d_ i

is the distance from an arbitrary point in the plane to the i-th vertex of a square and

R

displaystyle R

is the circumradius of the square, then[9]

d

1

4

+

d

2

4

+

d

3

4

+

d

4

4

4

+ 3

R

4

=

(

d

1

2

+

d

2

2

+

d

3

2

+

d

4

2

4

+

R

2

)

2

.

displaystyle frac d_ 1 ^ 4 +d_ 2 ^ 4 +d_ 3 ^ 4 +d_ 4 ^ 4 4 +3R^ 4 =left( frac d_ 1 ^ 2 +d_ 2 ^ 2 +d_ 3 ^ 2 +d_ 4 ^ 2 4 +R^ 2 right)^ 2 .

Coordinates and equations

x

+

y

= 2

displaystyle x+y=2

plotted on Cartesian coordinates.

The coordinates for the vertices of a square with vertical and horizontal sides, centered at the origin and with side length 2 are (±1, ±1), while the interior of this square consists of all points (xi, yi) with −1 < xi < 1 and −1 < yi < 1. The equation

max (

x

2

,

y

2

) = 1

displaystyle max(x^ 2 ,y^ 2 )=1

specifies the boundary of this square. This equation means "x2 or y2, whichever is larger, equals 1." The circumradius of this square (the radius of a circle drawn through the square's vertices) is half the square's diagonal, and equals

2

displaystyle scriptstyle sqrt 2

. Then the circumcircle has the equation

x

2

+

y

2

= 2.

displaystyle x^ 2 +y^ 2 =2.

Alternatively the equation

x − a

+

y − b

= r .

displaystyle leftx-aright+lefty-bright=r.

can also be used to describe the boundary of a square with center coordinates (a, b) and a horizontal or vertical radius of r. Construction The following animations show how to construct a square using a compass and straightedge. This is possible as 4 = 22, a power of two.

Square
Square
at a given circumcircle

Square
Square
at a given side length, right angle by using Thales' theorem

Square
Square
at a given diagonal

Symmetry

The dihedral symmetries are divided depending on whether they pass through vertices (d for diagonal) or edges (p for perpendiculars) Cyclic symmetries in the middle column are labeled as g for their central gyration orders. Full symmetry of the square is r12 and no symmetry is labeled a1.

The square has Dih4 symmetry, order 8. There are 2 dihedral subgroups: Dih2, Dih1, and 3 cyclic subgroups: Z4, Z2, and Z1. A square is a special case of many lower symmetry quadrilaterals:

a rectangle with two adjacent equal sides a quadrilateral with four equal sides and four right angles a parallelogram with one right angle and two adjacent equal sides a rhombus with a right angle a rhombus with all angles equal a rhombus with equal diagonals

These 6 symmetries express 8 distinct symmetries on a square. John Conway labels these by a letter and group order.[10] Each subgroup symmetry allows one or more degrees of freedom for irregular quadrilaterals. r8 is full symmetry of the square, and a1 is no symmetry. d4, is the symmetry of a rectangle and p4, is the symmetry of a rhombus. These two forms are duals of each other and have half the symmetry order of the square. d2 is the symmetry of an isosceles trapezoid, and p2 is the symmetry of a kite. g2 defines the geometry of a parallelogram. Only the g4 subgroup has no degrees of freedom but can seen as a square with directed edges. Squares inscribed in triangles Main article: Triangle
Triangle
§ Squares Every acute triangle has three inscribed squares (squares in its interior such that all four of a square's vertices lie on a side of the triangle, so two of them lie on the same side and hence one side of the square coincides with part of a side of the triangle). In a right triangle two of the squares coincide and have a vertex at the triangle's right angle, so a right triangle has only two distinct inscribed squares. An obtuse triangle has only one inscribed square, with a side coinciding with part of the triangle's longest side. The fraction of the triangle's area that is filled by the square is no more than 1/2. Squaring the circle Squaring the circle
Squaring the circle
is the problem, proposed by ancient geometers, of constructing a square with the same area as a given circle by using only a finite number of steps with compass and straightedge. In 1882, the task was proven to be impossible, as a consequence of the Lindemann–Weierstrass theorem
Lindemann–Weierstrass theorem
which proves that pi (π) is a transcendental number, rather than an algebraic irrational number; that is, it is not the root of any polynomial with rational coefficients. Non-Euclidean geometry In non-Euclidean geometry, squares are more generally polygons with 4 equal sides and equal angles. In spherical geometry, a square is a polygon whose edges are great circle arcs of equal distance, which meet at equal angles. Unlike the square of plane geometry, the angles of such a square are larger than a right angle. Larger spherical squares have larger angles. In hyperbolic geometry, squares with right angles do not exist. Rather, squares in hyperbolic geometry have angles of less than right angles. Larger hyperbolic squares have smaller angles. Examples:

Two squares can tile the sphere with 2 squares around each vertex and 180-degree internal angles. Each square covers an entire hemisphere and their vertices lie along a great circle. This is called a spherical square dihedron. The Schläfli symbol
Schläfli symbol
is  4,2 .

Six squares can tile the sphere with 3 squares around each vertex and 120-degree internal angles. This is called a spherical cube. The Schläfli symbol
Schläfli symbol
is  4,3 .

Squares can tile the Euclidean plane
Euclidean plane
with 4 around each vertex, with each square having an internal angle of 90°. The Schläfli symbol
Schläfli symbol
is 4,4 .

Squares can tile the hyperbolic plane with 5 around each vertex, with each square having 72-degree internal angles. The Schläfli symbol is  4,5 . In fact, for any n ≥ 5 there is a hyperbolic tiling with n squares about each vertex.

Crossed square

Crossed-square

A crossed square is a faceting of the square, a self-intersecting polygon created by removing two opposite edges of a square and reconnecting by its two diagonals. It has half the symmetry of the square, Dih2, order 4. It has the same vertex arrangement as the square, and is vertex-transitive. It appears as two 45-45-90 triangle with a common vertex, but the geometric intersection is not considered a vertex. A crossed square is sometimes likened to a bow tie or butterfly. the crossed rectangle is related, as a faceting of the rectangle, both special cases of crossed quadrilaterals.[11] The interior of a crossed square can have a polygon density of ±1 in each triangle, dependent upon the winding orientation as clockwise or counterclockwise. A square and a crossed square have the following properties in common:

Opposite sides are equal in length. The two diagonals are equal in length. It has two lines of reflectional symmetry and rotational symmetry of order 2 (through 180°).

It exists in the vertex figure of a uniform star polyhedra, the tetrahemihexahedron. Graphs

3-simplex
3-simplex
(3D)

The K4 complete graph is often drawn as a square with all 6 possible edges connected, hence appearing as a square with both diagonals drawn. This graph also represents an orthographic projection of the 4 vertices and 6 edges of the regular 3-simplex
3-simplex
(tetrahedron). See also

Geometry
Geometry
portal Mathematics portal

Cube Pythagorean theorem Square
Square
lattice Squircle Unit square

References

^ W., Weisstein, Eric. "Square". mathworld.wolfram.com. Retrieved 2017-12-12.  ^ Zalman Usiskin and Jennifer Griffin, "The Classification of Quadrilaterals. A Study of Definition", Information Age Publishing, 2008, p. 59, ISBN 1-59311-695-0. ^ "Problem Set 1.3". jwilson.coe.uga.edu. Retrieved 2017-12-12.  ^ Josefsson, Martin, "Properties of equidiagonal quadrilaterals" Forum Geometricorum, 14 (2014), 129-144. ^ "Maths is Fun - Can't Find It (404)". www.mathsisfun.com. Retrieved 2017-12-12.  ^ Chakerian, G.D. "A Distorted View of Geometry." Ch. 7 in Mathematical Plums (R. Honsberger, editor). Washington, DC: Mathematical Association of America, 1979: 147. ^ 1999, Martin Lundsgaard Hansen, thats IT (c). "Vagn Lundsgaard Hansen". www2.mat.dtu.dk. Retrieved 2017-12-12.  ^ " Geometry
Geometry
classes, Problem 331. Square, Point on the Inscribed Circle, Tangency Points. Math teacher Master Degree. College, SAT Prep. Elearning, Online math tutor, LMS". gogeometry.com. Retrieved 2017-12-12.  ^ Park, Poo-Sung. " Regular polytope
Regular polytope
distances", Forum Geometricorum 16, 2016, 227-232. http://forumgeom.fau.edu/FG2016volume16/FG201627.pdf ^ John H. Conway, Heidi Burgiel, Chaim Goodman-Strauss, (2008) The Symmetries of Things, ISBN 978-1-56881-220-5 (Chapter 20, Generalized Schaefli symbols, Types of symmetry of a polygon pp. 275-278) ^ Wells, Christopher J. "Quadrilaterals". www.technologyuk.net. Retrieved 2017-12-12. 

External links

Wikimedia Commons has media related to Squares (geometry).

Animated course (Construction, Circumference, Area) Weisstein, Eric W. "Square". MathWorld.  Definition and properties of a square With interactive applet Animated applet illustrating the area of a square

v t e

Polygons

Regular List

1–10 sides

Monogon Digon Triangle

Equilateral Isosceles

Quadrilateral

Square Rectangle Rhombus Parallelogram Trapezoid Kite

Pentagon Hexagon Heptagon Octagon Nonagon
Nonagon
(Enneagon) Decagon

11–20 sides

Hendecagon Dodecagon Tridecagon Tetradecagon Pentadecagon Hexadecagon Heptadecagon Octadecagon Enneadecagon Icosagon

21–100 sides (selected)

Icosidigon
Icosidigon
(22) Icositetragon
Icositetragon
(24) Icosihexagon
Icosihexagon
(26) Icosioctagon
Icosioctagon
(28) Triacontagon
Triacontagon
(30) Triacontadigon
Triacontadigon
(32) Triacontatetragon
Triacontatetragon
(34) Tetracontagon
Tetracontagon
(40) Tetracontadigon
Tetracontadigon
(42) Tetracontaoctagon
Tetracontaoctagon
(48) Pentacontagon
Pentacontagon
(50) Hexacontagon
Hexacontagon
(60) Hexacontatetragon
Hexacontatetragon
(64) Heptacontagon
Heptacontagon
(70) Octacontagon
Octacontagon
(80) Enneacontagon
Enneacontagon
(90) Enneacontahexagon
Enneacontahexagon
(96) Hectogon
Hectogon
(100)

>100 sides

120-gon 257-gon 360-gon Chiliagon
Chiliagon
(1,000) Myriagon
Myriagon
(10,000) 65537-gon Megagon
Megagon
(1,000,000) Apeirogon
Apeirogon
(∞)

Star polygons (5–12 sides)

Pentagram Hexagram Heptagram Octagram Enneagram Decagram Hendecagram Dodecagram

v t e

Fundamental convex regular and uniform polytopes in dimensions 2–10

Family An Bn I2(p) / Dn E6 / E7 / E8 / F4 / G2 Hn

Regular polygon Triangle Square p-gon Hexagon Pentagon

Uniform polyhedron Tetrahedron Octahedron
Octahedron
• Cube Demicube

Dodecahedron • Icosahedron

Uniform 4-polytope 5-cell 16-cell
16-cell
• Tesseract Demitesseract 24-cell 120-cell
120-cell
• 600-cell

Uniform 5-polytope 5-simplex 5-orthoplex
5-orthoplex
• 5-cube 5-demicube

Uniform 6-polytope 6-simplex 6-orthoplex
6-orthoplex
• 6-cube 6-demicube 122 • 221

Uniform 7-polytope 7-simplex 7-orthoplex
7-orthoplex
• 7-cube 7-demicube 132 • 231 • 321

Uniform 8-polytope 8-simplex 8-orthoplex
8-orthoplex
• 8-cube 8-demicube 142 • 241 • 421

Uniform 9-polytope 9-simplex 9-orthoplex
9-orthoplex
• 9-cube 9-demicube

Uniform 10-polytope 10-simplex 10-orthoplex
10-orthoplex
• 10-cube 10-demicube

Uniform n-polytope n-simplex n-orthoplex • n-cube n-demicube 1k2 • 2k1 • k21 n-pentagonal polytope

Topics: Polytope
Polytope
families • Regular polytope
Regular polytope
• List of regular polytopes and compounds

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