Quadrilateral is a geometric form that is composed of 4 points (vertices) sequentially joined by straight line segments (sides). We discover the etymology of the word in S. Schwartzman"s The Words of Mathematics:

quadrilateral (noun, adjective): the first element is from Latin quadri- "four" from the Indo-European root kwetwer- "4." The second element is from Latin latus, stem later-, "side," of unrecognized prior beginning. A quadrilateral is a four-sided polygon. The Latin term is a partial translation of Greek tetragon, literally "4 angles," because a closed number with four angles likewise has actually four sides. Although we usage words favor pentagon and also polygon, the term quadrilateral has entirely reinserted tetragon.

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The rarely used term quadrangle has exactly the exact same meaning as quadrilateral, but the 2 related terms -- finish quadrangle and also finish quadrilateral -- explain essentially various configurations.

A quadrilateral may be convex or concave (watch the diagram listed below.) A quadrilateral that is concave has actually an angle exceeding 180o. In either case, the quadrilateral is basic, which implies that the 4 sides of the quadrilateral just accomplish at the vertices, two at a time. So that 2 non-adjacent sides perform not cross. A quadrilateral that is not easy is additionally recognized as self-intersecting to suggest that a pair of his non-surrounding sides intersect. The point of interarea of the sides is not considered a vertex of the quadrilateral.

The shapes of elementary geomeattempt are invariably convex. Starting via the the majority of regular quadrilateral, namely, the square, we shall define other forms by relaxing its properties.

A square is a quadrilateral via all sides equal and also all angles additionally equal. Angles in any type of quadrilateral include approximately 360°. It follows that, in a square, all angles measure 90°. An equiangular quadrilateral, i.e. the one via all angles equal is a rectangle. All angles of a rectangle equal 90°. An equilateral quadrilateral, i.e. the one with all sides equal, is a rhombus.

In a square, rectangle, or rhombus, the opposite side lines are parallel. A quadrilateral through the opposite side lines parallel is known as a parallelogram. If just one pair of opposite sides is required to be parallel, the form is a trapezoid. A trapezoid, in which the non-parallel sides are equal in size, is called isosceles. A quadrilateral through 2 sepaprice pairs of equal nearby sides is typically dubbed a kite. However, if the kite is concave, a dart is an extra proper term. Kite and also dart are examples of orthodiagonal quadrilaterals, i.e. quadrilaterals through perpendicular diagonals. A square and also a rhombus are additionally specific instances of this class.

The four vertices of a quadrilateral may be concyclic, i.e., lie on the same circle. In this case, the quadrilateral is known as circumscritptible or, much easier, cyclic. If a quadrilateral admits an incircle that touches all 4 of its sides (or more primarily, side lines), it is well-known as inscriptible. A quadrilateral, both cyclic and inscriptible, is bicentric.

The diagram below (which is a alteration of one from wikipedia.org) summarize the partnership between assorted kinds of quadrilaterals: The applet listed below illustrates the properties of assorted quadrilaterals. In the applet, one can drag the vertices and the sides of the quadrilateral. You can display its diagonals, angle bisectors and also the perpendicular bisectors of its sides. With these props, it"s a straightforward matter to observe eextremely single sort of quadrilateral, via a possible exemption of bicentric. Which, too, is not overly difficult if you first obtain an isosceles trapezoid.)

### This applet needs Sun"s Java VM 2 which your internet browser may perceive as a popup. Which it is not. If you desire to view the applet work-related, visit Sun"s webwebsite at https://www.java.com/en/download/index.jsp, download and also install Java VM and also reap the applet.

 What if applet does not run?

As in the classification of triangles, the meanings may be either inclusive or exclusive. For example, trapezoid may be characterized inclusively as a quadrilateral through a pair of parallel oppowebsite sides, or solely as a quadrilateral with exactly one such pair. In the former situation, parallelogram is a trapezoid, in the last, it is not. Similarly, a square may or might not be a rectangle or a rhombus. My preference is with the inclusive approach. For, I"d favor to think of a square as a rhombus through ideal angles, or as a rectangle with all four sides equal.

Here is a list of all the properties of quadrilaterals that we have actually stated together with the classes of the quadrilaterals that possess those properties:

 Property Quadrilaterals Orthodiagonal Kite, Dart, Rhombus, Square Cyclic Square, Rectangle, Isosceles Trapezoid Inscriptible Kite, Dart, Rhombus, Square Having 2 parallel sides Rhombus, Square, Rectangle, Parallelogram, Trapezoid Having 2 pairs of parallel sides Rhombus, Square, Rectangle, Parallelogram Equilateral Rhombus, Square Equiangular Rectangle, Square

Orthodiagonal or inscriptible parallelogram is a rhombus; cyclic parallelogram is a rectangle. In specific, a parallelogram through equal diagonals is necessarily a rectangle. And not to forobtain, eextremely easy quadrilateral tiles the airplane.

A basic quadrilateral via two pairs of equal oppowebsite angles is a parallelogram. (Since then the opposite sides are parallel.) A straightforward quadrilateral via 2 pairs of equal oppowebsite sides is a parallelogram. (Due to the fact that of SSS once you attract one of the diagonals.)There is a basic quadrilateral with two pairs of equal sides: a kite (or a dart). It does have actually a pair of opposite equal angles.

Nathan Bowler suggested a basic building and construction of a quadrilateral through a pair of equal oppowebsite sides and also a pair of equal oppowebsite angles which is not necessarily a parallelogram (tbelow is a dynamic illustration):

Let ABC be isosceles through AB = AC. Pick D on BC. Let C" be the reflection of C in the perpendicular bisector of AD. ABDC" has 2 opposite sides the exact same size and 2 oppowebsite angles equal but is not a parallelogram if D isn"t the midsuggest of AB. This building and construction gives all such quadrilaterals.

For an isosceles trapezoid ABCD through AB = CD, the quadrilateral ABDC has actually a pair of equal opposite sides and also two pairs of equal opposite angles.

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## References

S. Schwartzmale, The Words of Mathematics, MAA, 1994