Trapezoid

Hutton’s mistake in 1795[4]

Ancient Greek mathematician Euclid defined five types of quadrilateral, of which four had two sets of parallel sides (known in English as square, rectangle, rhombus and rhomboid) and the last did not have two sets of parallel sides – a τραπέζια (trapezia[5] literally "a table", itself from τετράς (tetrás), "four" + πέζα (péza), "a foot; end, border, edge").[6]

Two types of trapezia were introduced by Proclus (412 to 485 AD) in his commentary on the first book of Euclid's Elements:[4][7]

• one pair of parallel sides – a trapezium (τραπέζιον), divided into isosceles (equal legs) and scalene (unequal) trapezia
• no parallel sides – trapezoid (τραπεζοειδή, trapezoeidé, literally trapezium-like (εἶδος means "resembles"), in the same way as cuboid means cube-like and rhomboid means rhombus-like)

All European languages follow Proclus's structure[7][8] as did English until the late 18th century, until an influential mathematical dictionary published by Charles Hutton in 1795 supported without explanation a transposition of the terms. This mistake was corrected in British English in about 1875, but was retained in American English into the modern day.[4]

Type	Sets of parallel sides	Image	Original terminology			Modern terminology
			Euclid (Definition 22)	Proclus (Definitions 30-34, quoting Posidonius)	Euclid / Proclus definition	British English (and European languages)	American English
Parallelogram	2		ῥόμβος (rhombos)		equilateral but not right-angled	Rhombus		Trapezoid (inclusive)
			ῥομβοειδὲς (rhomboides)		opposite sides and angles equal to one another but not equilateral nor right-angled	Rhomboid (colloquially Parallelogram)
Non-parallelogram	1		τραπέζια (trapezia)	τραπέζιον ἰσοσκελὲς (trapezion isoskelés)	Two parallel sides, and a line of symmetry	Isoceles Trapezium	Isoceles Trapezoid
				τραπέζιον σκαληνὸν (trapezion skalinón)	Two parallel sides, and no line of symmetry	Trapezium	Trapezoid (exclusive)
	0			τραπέζοειδὲς (trapezoides)	No parallel sides	Trapezoid	Trapezium

The shape is often called an irregular quadrilateral.[9][10]

There is some disagreement whether parallelograms, which have two pairs of parallel sides, should be regarded as trapezoids. Some define a trapezoid as a quadrilateral having only one pair of parallel sides (the exclusive definition), thereby excluding parallelograms.[11] Others[12] define a trapezoid as a quadrilateral with at least one pair of parallel sides (the inclusive definition[13]), making the parallelogram a special type of trapezoid. The latter definition is consistent with its uses in higher mathematics such as calculus. This article uses the inclusive definition and considers parallelograms as special cases of a trapezoid. This is also advocated in the taxonomy of quadrilaterals.

Under the inclusive definition, all parallelograms (including rhombuses, squares and non-square rectangles) are trapezoids. Rectangles have mirror symmetry on mid-edges; rhombuses have mirror symmetry on vertices, while squares have mirror symmetry on both mid-edges and vertices.

Trapezoid special cases. The orange figures also qualify as parallelograms.

A right trapezoid (also called right-angled trapezoid) has two adjacent right angles.[12] Right trapezoids are used in the trapezoidal rule for estimating areas under a curve.

An acute trapezoid has two adjacent acute angles on its longer base edge, while an obtuse trapezoid has one acute and one obtuse angle on each base.

An isosceles trapezoid is a trapezoid where the base angles have the same measure. As a consequence the two legs are also of equal length and it has reflection symmetry. This is possible for acute trapezoids or right trapezoids (rectangles).

A parallelogram is a trapezoid with two pairs of parallel sides. A parallelogram has central 2-fold rotational symmetry (or point reflection symmetry). It is possible for obtuse trapezoids or right trapezoids (rectangles).

A tangential trapezoid is a trapezoid that has an incircle.

A Saccheri quadrilateral is similar to a trapezoid in the hyperbolic plane, with two adjacent right angles, while it is a rectangle in the Euclidean plane. A Lambert quadrilateral in the hyperbolic plane has 3 right angles.

Four lengths a, c, b, d can constitute the consecutive sides of a non-parallelogram trapezoid with a and b parallel only when[14]

${\displaystyle \displaystyle |d-c|<|b-a|<d+c.}$

The quadrilateral is a parallelogram when ${\displaystyle d-c=b-a=0}$, but it is an ex-tangential quadrilateral (which is not a trapezoid) when ${\displaystyle |d-c|=|b-a|\neq 0}$.[15]^{: p. 35}

general trapezoid/trapezium:
parallel sides: ${\displaystyle a,\,b}$ with ${\displaystyle a<b}$
legs: ${\displaystyle c,\,d}$
diagonals: ${\displaystyle q,\,p}$
midsegment: ${\displaystyle m}$
height/altitude: ${\displaystyle h}$

trapezoid/trapezium with opposing triangles ${\displaystyle S,\,T}$ formed by the diagonals

Given a convex quadrilateral, the following properties are equivalent, and each implies that the quadrilateral is a trapezoid:

• It has two adjacent angles that are supplementary, that is, they add up to 180 degrees.
• The angle between a side and a diagonal is equal to the angle between the opposite side and the same diagonal.
• The diagonals cut each other in mutually the same ratio (this ratio is the same as that between the lengths of the parallel sides).
• The diagonals cut the quadrilateral into four triangles of which one opposite pair have equal areas.[15]^: Prop.5
• The product of the areas of the two triangles formed by one diagonal equals the product of the areas of the two triangles formed by the other diagonal.[15]^: Thm.6
• The areas S and T of some two opposite triangles of the four triangles formed by the diagonals satisfy the equation

${\displaystyle {\sqrt {K}}={\sqrt {S}}+{\sqrt {T}},}$

where K is the area of the quadrilateral.[15]^: Thm.8

• The midpoints of two opposite sides and the intersection of the diagonals are collinear.[15]^: Thm.15
• The angles in the quadrilateral ABCD satisfy ${\displaystyle \sin A\sin C=\sin B\sin D.}$[15]^{: p. 25}
• The cosines of two adjacent angles sum to 0, as do the cosines of the other two angles.[15]^{: p. 25}
• The cotangents of two adjacent angles sum to 0, as do the cotangents of the other two adjacent angles.[15]^{: p. 26}
• One bimedian divides the quadrilateral into two quadrilaterals of equal areas.[15]^{: p. 26}
• Twice the length of the bimedian connecting the midpoints of two opposite sides equals the sum of the lengths of the other sides.[15]^{: p. 31}

Additionally, the following properties are equivalent, and each implies that opposite sides a and b are parallel:

• The consecutive sides a, c, b, d and the diagonals p, q satisfy the equation[15]^: Cor.11

${\displaystyle p^{2}+q^{2}=c^{2}+d^{2}+2ab.}$

• The distance v between the midpoints of the diagonals satisfies the equation[15]^: Thm.12

${\displaystyle v={\frac {|a-b|}{2}}.}$

The midsegment (also called the median or midline) of a trapezoid is the segment that joins the midpoints of the legs. It is parallel to the bases. Its length m is equal to the average of the lengths of the bases a and b of the trapezoid,[12]

${\displaystyle m={\frac {a+b}{2}}.}$

The midsegment of a trapezoid is one of the two bimedians (the other bimedian divides the trapezoid into equal areas).

The height (or altitude) is the perpendicular distance between the bases. In the case that the two bases have different lengths (a ≠ b), the height of a trapezoid h can be determined by the length of its four sides using the formula[12]

${\displaystyle h={\frac {\sqrt {(-a+b+c+d)(a-b+c+d)(a-b+c-d)(a-b-c+d)}}{2|b-a|}}}$

where c and d are the lengths of the legs.

The area K of a trapezoid is given by[12]

${\displaystyle K={\frac {a+b}{2}}\cdot h=mh}$

where a and b are the lengths of the parallel sides, h is the height (the perpendicular distance between these sides), and m is the arithmetic mean of the lengths of the two parallel sides. In 499 AD Aryabhata, a great mathematician-astronomer from the classical age of Indian mathematics and Indian astronomy, used this method in the Aryabhatiya (section 2.8). This yields as a special case the well-known formula for the area of a triangle, by considering a triangle as a degenerate trapezoid in which one of the parallel sides has shrunk to a point.

The 7th-century Indian mathematician Bhāskara I derived the following formula for the area of a trapezoid with consecutive sides a, c, b, d:

${\displaystyle K={\frac {1}{2}}(a+b){\sqrt {c^{2}-{\frac {1}{4}}\left((b-a)+{\frac {c^{2}-d^{2}}{b-a}}\right)^{2}}}}$

where a and b are parallel and b > a.[16] This formula can be factored into a more symmetric version[12]

${\displaystyle K={\frac {a+b}{4|b-a|}}{\sqrt {(-a+b+c+d)(a-b+c+d)(a-b+c-d)(a-b-c+d)}}.}$

When one of the parallel sides has shrunk to a point (say a = 0), this formula reduces to Heron's formula for the area of a triangle.

Another equivalent formula for the area, which more closely resembles Heron's formula, is[12]

${\displaystyle K={\frac {a+b}{|b-a|}}{\sqrt {(s-b)(s-a)(s-b-c)(s-b-d)}},}$

where ${\displaystyle s={\tfrac {1}{2}}(a+b+c+d)}$ is the semiperimeter of the trapezoid. (This formula is similar to Brahmagupta's formula, but it differs from it, in that a trapezoid might not be cyclic (inscribed in a circle). The formula is also a special case of Bretschneider's formula for a general quadrilateral).

From Bretschneider's formula, it follows that

${\displaystyle K={\sqrt {{\frac {(ab^{2}-a^{2}b-ad^{2}+bc^{2})(ab^{2}-a^{2}b-ac^{2}+bd^{2})}{4(b-a)^{2}}}-\left({\frac {c^{2}+d^{2}-a^{2}-b^{2}}{4}}\right)^{2}}}.}$

The line that joins the midpoints of the parallel sides, bisects the area.

The lengths of the diagonals are[12]

${\displaystyle p={\sqrt {\frac {ab^{2}-a^{2}b-ac^{2}+bd^{2}}{b-a}}},}$

${\displaystyle q={\sqrt {\frac {ab^{2}-a^{2}b-ad^{2}+bc^{2}}{b-a}}}}$

where a is the short base, b is the long base, and c and d are the trapezoid legs.

If the trapezoid is divided into four triangles by its diagonals AC and BD (as shown on the right), intersecting at O, then the area of ${\displaystyle \triangle }$ AOD is equal to that of ${\displaystyle \triangle }$ BOC, and the product of the areas of ${\displaystyle \triangle }$ AOD and ${\displaystyle \triangle }$ BOC is equal to that of ${\displaystyle \triangle }$ AOB and ${\displaystyle \triangle }$ COD. The ratio of the areas of each pair of adjacent triangles is the same as that between the lengths of the parallel sides.[12]

Let the trapezoid have vertices A, B, C, and D in sequence and have parallel sides AB and DC. Let E be the intersection of the diagonals, and let F be on side DA and G be on side BC such that FEG is parallel to AB and CD. Then FG is the harmonic mean of AB and DC:[17]

${\displaystyle {\frac {1}{FG}}={\frac {1}{2}}\left({\frac {1}{AB}}+{\frac {1}{DC}}\right).}$

The line that goes through both the intersection point of the extended nonparallel sides and the intersection point of the diagonals, bisects each base.[18]

The center of area (center of mass for a uniform lamina) lies along the line segment joining the midpoints of the parallel sides, at a perpendicular distance x from the longer side b given by[19]

${\displaystyle x={\frac {h}{3}}\left({\frac {2a+b}{a+b}}\right).}$

The center of area divides this segment in the ratio (when taken from the short to the long side)[20]^{: p. 862}

${\displaystyle {\frac {a+2b}{2a+b}}.}$

If the angle bisectors to angles A and B intersect at P, and the angle bisectors to angles C and D intersect at Q, then[18]

${\displaystyle PQ={\frac {|AD+BC-AB-CD|}{2}}.}$

• Frustum, a solid having trapezoidal faces
• Polite number, also known as a trapezoidal number
• Wedge, a polyhedron defined by two triangles and three trapezoid faces.

• D. Fraivert, A. Sigler and M. Stupel : Common properties of trapezoids and convex quadrilaterals

• "Trapezium" at Encyclopedia of Mathematics
• Weisstein, Eric W. "Right trapezoid". MathWorld.
• Trapezoid definition   Area of a trapezoid   Median of a trapezoid With interactive animations
• Trapezoid (North America) at elsy.at: Animated course (construction, circumference, area)
• Trapezoidal Rule on Numerical Methods for Stem Undergraduate
• Autar Kaw and E. Eric Kalu, Numerical Methods with Applications, (2008)