Talk:Quadrilateral/Archive 1

Archive 1

Picture

The picture at the bottom of this article is wrong. It says that a parallelogram is always an isosceles trapezoid. A non-rectangular parallelogram as rotational symmetry; an isosceles trapezoid has reflection symmetry. These mismatch so these are 2 separate kinds of quadrilaterals where the rectangle is the special case of both. Any comments on this?? 66.245.28.124 22:27, 19 Oct 2004 (UTC)

See above. The diagram is fixed but the old version is still in Wikipedia's image cache. Gdr 22:40, 2004 Oct 19 (UTC)

also....stuff like this are really boring.....really really boring.

HAPPY CHRISTMAS TO ALL RAMADANS!

Diagonal section difficult to read. Why were section names deleted??

Bring back the section names that were deleted on 06:34, 9 April 2012‎ !!! — Preceding unsigned comment added by 2.123.160.75 (talk)

See

cross-quadrilateral --> autosecant quadrilateral — Preceding unsigned comment added by Tarpuq (talk • contribs) 18:12, 10 February 2018 (UTC)

Square in the diagram

The square in the diagram doesn't look like a perfect square as I can see by rotating it. Georgia guy 16:45, 15 May 2005 (UTC)

Which diagram? Gdr June 28, 2005 08:28 (UTC)

arrowhead/chevron

Arrowhead: the forgotten quadrilateral. From memory this is the shape that adorned the uniforms of the crew of the USS Enterprise in the original Star Trek - though perhaps a more formal definition is called for. I open the bidding with "a quadrilateral with an internal reflex angle". Why is the arrowhead grievously overlooked (it's not in Encarta for instance)? Is the correct term Arrowhead or Chevron or is this the nameless as well as the forgotten quadrilateral? On the face of it this shape deserves a separate mention alongside rectangle, rhombus etc. —The preceding unsigned comment was added by Brainfood (talk • contribs) .

The reflex angle is covered by "concave quadrilateral". I assume you mean a sort of concave kite with pairs of equal sides. --Henrygb 12:20, 24 January 2006 (UTC)

Template

Any opinions on a template for quadrilaterals?? Georgia guy 16:33, 21 June 2006 (UTC)

Quadrilatère complet

hello, I am french and my english is not very good. I have just a question: how do you call this figure (in french fr:quadrilatère complet). Thank you. HB

We call it a complete quadrilateral in English too.--Syd Henderson 03:20, 19 August 2006 (UTC)

added "not" a square, to definition of oblong (rectangle)

I went ahead and made this change in "The square as a rectangle" section of article., as I am pretty sure it was a simple mistake (i.e., "A rectangle that is not a square is an oblong." was the intended statement). I think more qualification might be necessary, as the term 'oblong' is only relevant if a square is included in the definition of rectangle (which I think among mathematicians it always is), but I sink pretty fast in these terminological quagmires, especially if I am making any attempt to cultivate one. Jauntymcd 11:44, 20 March 2007 (UTC)

Complex quadrilateral

In Coxeter's book Regular complex polytopes, a complex quadrilateral is a quadrilateral in the unitary plane, which is quite a different beast - it involves complex numbers and is kind of four-dimensional (two real dimensions and two imaginary ones). So where does the use of "complex" meaning self-intersecting (not simple) come from? Can somebody give a reference? -- Steelpillow 22:30, 5 July 2007 (UTC)

I'm sure it is talking about this meaning: Complex polygon#Computer graphics. Tom Ruen 22:45, 5 July 2007 (UTC)

Yes I am sure it is. And that page doesn't have an explanation or a reference for it either. In fact, I added the heading on 23 December 2006, in order to distance the content from the way Coxeter used the term. I don't actually know whether the "non-convex" usage is confined to computer graphics. I noticed that its use in this context was widespread on t'Internet, and again I don't know whether this is because it always has been or whether this presence has grown entirely due to people copying from Wikipedia. I had hoped that someone with better knowledge would either correct or confirm my heading (and add a reference!), but in six months - nothing. -- Steelpillow 22:03, 8 July 2007 (UTC)

How has this page helped you?

This page helped me in my studies for a math test.--Pure-intellect (talk) 19:13, 8 January 2008 (UTC)

Brahmagupta's formula

Shouldn't there be mention of Brahmagupta's formula?  Laptopdude  Talk  21:36, 25 April 2008 (UTC)

Done. 129.199.98.65 (talk) 14:41, 21 November 2008 (UTC)

Quadrilateral are a funny shape but the answer to most of them is mc=2 —Preceding unsigned comment added by 79.73.121.12 (talk) 20:15, 23 June 2008 (UTC)

a Quadrilateral is a funny shape but the answer to most of them is mc=2 —Preceding unsigned comment added by Dsilvetser (talk • contribs) 20:16, 23 June 2008 (UTC)

Trapezoid/trapezium

I could have sworn that the two sides parallel ones are called "trapezoids". --BlackGriffen

And you would have been right. I've made the change. Vicki Rosenzweig

The article thus far lists only cyclic quadrilaterals; can someone who's done geometry within the last 20 years fix this? Vicki Rosenzweig (yes, me again)

The trapezoid is three dimensional.

--- User:Karl Palmen

I've always known a trapezoid to be two dimensional, and Trapezium to be a trapezoidal object associated with astronomy. -- Olof

Wolfram's MathWorld [1] gives: "There are two common definitions of the trapezium. The American definition is a quadrilateral with no parallel sides. The British definition for a trapezium is a quadrilateral with two sides parallel." and "The trapezoid is equivalent to the British definition of trapezium." -- Olof

Could a British speaker confirm this? Also, is "trapezoid" used in Britain, and if so, in what meaning? Maybe this should be added to American and British English Differences. AxelBoldt

I'm not sure about "trapezoid", but a trapezium definitely has two parallel sides in British usage. --Zundark, 2002 Mar 11

I second Zundark. BTW, to whoever did the picture: it's excellent! -- Tarquin 13:38 Aug 20, 2002 (PDT)

American and British English Differences is a page of general differences. This specific difference belongs and already is on List of words having different meanings in British and American English. -- Smjg 15:14, 26 Jul 2004 (UTC)

My UK English dictionary (The Concise English Dictionary, 1984 edition but copyright 1968 for some reason) lists

Trapezium - n. A quadrilateral figure no or only two sides of which are parallel

Trapezoid - n. A quadrilateral only two or no two of whose sides are parallel

which I think makes both the same -- Ablonus (talk) 10:35, 24 November 2008 (UTC)

Yes, both words have been used with both meanings in the past in British usage. Here is the history, for what it's worth:

Euclid used the word trapezium (in Greek, of course) to described all quadrilaterals more general than the parallelogram. Marinus Proclus (410 to 485 AD approx) in his Commentary on the first book of Euclid’s Elements invented the word translated trapezoid to refer to a general quadrilateral having no special properties. In 1788, Taylor’s translation of this commentary was published, including the sentence: “Of non-parallelograms, some have only two parallel sides, ... others have none of their sides parallel. And those are called Trapeziums, but these Trapezoids.”

In his Mathematical and Philosophical Dictionary, published in 1795, Charles Hutton seems to have misunderstood the order of Proclus’ sentence, for he defines Trapezium as “a plane figure contained under four right lines, of which both the opposite pairs are not parallel. When this figure has two of its sides parallel to each other, it is sometimes called a trapezoid”. This usage seems to have been adopted by both British and American mathematicians. Towards the end of the 1800s, British usage seems to have reverted to Proclus’ original definitions, and the term trapezium is regularly used for a quadrilateral with one pair of parallel sides throughout Britain. Dictionaries published around this time also have trapezoid defined as a quadrilateral having no parallel sides, but this usage seems to have died out in the UK (except in dictionaries where historic usage is recorded). The OED records this definition, saying only “Trapezoid (no sides parallel sense) Often called by English writers in the 19th century”. The term "Trapezoid" is now seldom if ever used by British mathematicians and is not normally taught in British schools. Modern usage is usually adjectival, and refers to parallel sides. Dbfirs 11:31, 24 November 2008 (UTC)

The article, under Convex Quadrilaterals and maybe elsewhere, is confusing when it tries to describe the differing nomenclatures used in the US and the UK. Looking at the history as posted above it's not hard to see why. Nevertheless it should be possible, IMHO, to make the article easier to read. How? Some options: 1) standardise the way the bullet points list the info, 2) include the pictures with the bullets to which they relate, 3) express as a table. Any others? Ablonus (talk) 12:14, 11 December 2008 (UTC)

Error in chart?

According to the Wikipedia entry, Kites are not necessarily convex. 81.129.140.103 (talk) 17:53, 23 April 2009 (UTC)

I've added a note to clarify this. I will also edit the entry at kite to clarify that most people exclude the arrowhead or dart from the definition of kite. The chart reflects the usual definition in UK schools. Is it different elsewhere? Dbfirs 08:19, 25 April 2009 (UTC)

Contradictory statements

The article says both

• Trapezium (in British) or trapezoid (American English): no sides are parallel.
• Trapezium (British English) or trapezoid (Amer.): two opposite sides are parallel.

I have no idea which of the two is correct, so I'm not changing anything, but clearly one of the two has the Brit and American names swapped.99.130.29.223 07:46, 10 November 2007 (UTC)

(This error was corrected long ago) Dbfirs 07:32, 10 September 2009 (UTC)

UK / US usage

I was always taught at school (in the UK in the seventies and eighties) that a trapezium has one pair of parallel sides and a trapezoid has none. Is this a difference is UK / US usage, i.e. that a UK trapezium is the same as a US trapezoid and vice versa? The UK meaning of trapezoid is missing from the article, and Wiktionary lists it as "obsolete". Can anyone clarify this? HairyDan (talk) 12:18, 23 March 2008 (UTC)

The term trapezoid is not normally taught in UK schools because of its confused meaning (which has changed twice in British usage). Most schools just call the shape an irregular quadrilateral, but your teacher was using the genuine sense in which the word trapezoid was introduced by Proclus who lived around 410-485 AD. Many Commonwealth countries now use trapezoid as a synonym for the British Trapezium, and all the British websites I have been able to find use it either to mean the same as the British trapezium or a 3-D shape with some parallel sides. I would be very interested to know whether you have ever seen the trapezoid has no parallel sides used in print anywhere (other than copies of old dictionaries). I have not been able to find a single actual usage. Dbfirs 16:16, 22 November 2008 (UTC)

See also the comments which are currently under section 2 of this talk page: Trapezoid/trapezium. Ablonus (talk) 12:16, 11 December 2008 (UTC)

I keep seeing words about US usage regarding the term "trapezium". I can say this is not a term I have ever encountered in any US school. I've searched my collection of math books and found two books referencing "trapezium": a geometry book published in 1865 and recopyrighted in 1884, and a geometry book published in 1893. I checked two of my more contemporary books published in 1959 and 1991, the latter a current Honors Geometry book for my school district and did not find the term trapezium. The term US student's learn for what is being called trapezium is quadrilateral. In the US, quadrilaterals are defined as a four-sided polygon and depicted with no special properties. They just have four sides, four angles, two diagonals and sum of interior angles measuring 360 degrees. When you subract the special quadrilaterals from the entire set, these are what remain and their shape is used to represent the entire set. I feel confident that this term has not been taught in US schools in the past 50 years. I can't say with certainty beyond that, but my guess is that it would extend to the past 100 years. At my first opportunity, I will take a trip to my local used bookstore and check out the math section to determine if I can find any books referencing "trapezium". However, these books will be recently published textbooks or self-teaching books. Based on what another editor indicated, I would guess that "trapezoid" is more commonly found in the UK than "trapezium" is found in the US. The following site is a basic snapshot regarding how quadrilaterals are taught in the US: www.math.com/school/subject3/lessons/S3U2L3DP.html  JackOL31 (talk) 02:31, 10 September 2009 (UTC)

For once, I agree with you 100%! Why would US schools want to use the term "trapezium"? The usage is probably long obsolete. Similarly, when "trapezoid" is used in the UK it means either exactly the same as it does in the US, or it refers to a 3-D solid with faces having a pair of parallel sides. The sooner we get rid of confusing obsolete usages, the better! Dbfirs 07:29, 10 September 2009 (UTC)

...(later) Even Mathworld [2] now has a more considered approach. I propose that we ask the originator of Image:Quadrilaterals.svg for permission to remove the old American drawing of a trapezium. Dbfirs 07:53, 10 September 2009 (UTC)

Makes sense to me, both the drawing and the bullet point. Some might object and want the shape there with some description such a quadrilateral or general quadrilateral (Am) and irregular quadrilateral (Br). JackOL31 (talk) 02:22, 11 September 2009 (UTC)

Yes, perhaps better just to rename the shape. I'm happy with "general quadrilateral" (inclusive). Dbfirs 11:52, 11 September 2009 (UTC)

So how does one go about changing the world? Even the site I mentioned says that "trapezium" is the Br usage for "trapezoid". As you stated earlier, those usages are most likely obsolete. JackOL31 (talk) 21:18, 3 October 2009 (UTC)

Yes, everyone agrees that the shape with two parallel sides is called a trapezoid in the USA and a trapezium in the UK. Other English-speaking countries often allow both words to refer to the same shape. The confusion goes back to Charles Hutton's Mathematical and Philosophical Dictionary published in 1795, and appears to be a misreading of Taylor's 1788 translation of Proclus. My preference would be to avoid the use of either word for either a general quadrilateral (including all others) or an irregular quadrilateral (excluding any regularity, even parallel sides). These mean the same world-wide don't they? Dbfirs 08:42, 20 November 2009 (UTC)

I would say that the term general quadrilateral also means excluding any regularity. If I wanted to include all others, I would simply say quadrilateral. I prefer positive terms over negative terms where possible and I would say general quadrilateral and irregular quadrilateral are the same thing. I would suggest that the terms be listed in the following order: Parallelogram, Rhomboid, Rhombus, Rectangle, Oblong and Square. One could then indicate that a Rhomboid is a parallelogram that is none of the below shapes. Similar statement for Oblong. For many, Oblong is not used as a geometrical term. Regardless, it needs to be noted that Rhomboid is no longer in use in NAm (I would call it a general parallelogram). Applying the same idea to "Convex Quads - Other", the order of the list shbe Trapezium/Trapezoid, Trapezoid/Trapezium, Isosceles Traps, Kite and then the remaining shapes. As before, Trapezium is no longer in use in NAm (I would call it a general quadrilateral/irregular quadrilateral). Of course, I am not speaking for the usage/nonusage of the "Br" Trapezoid and Trapezium. JackOL31 (talk) 19:15, 21 November 2009 (UTC)

I agree except that I use general to mean not assuming any regularity, but not precluding it. I'm happy just to say quadrilateral. I also agree that rhomboid is not in common use. Oblong is only an informal term, not used in formal geometry. The diagram at the end of the article accurately summarises the heirarchy. Dbfirs 16:12, 23 November 2009 (UTC)

Taxonomy is wrong, or is it?

A parallelogram is a special case of a trapezium, not an isosceles trapezium. An isosceles trapezium has adjacent angles equal and opposite angles generally unequal; a parallelogram has opposite angles equal and adjacent sides generally unequal.

In precisely the same way, a rhombus is not a special case of a 3-sides-equal trapezium, only of kite and parallelogram.

But a square is a special case of a 3-sides-equal trapezium.

-- Smjg 15:14, 26 Jul 2004 (UTC)

Meanwhile, I've put back the ASCII taxonomy graph and expanded on it a bit. If anyone would like to do this one up as an image, I'll leave it up to you.... -- Smjg 14:05, 27 Sep 2004 (UTC)

Diagram now fixed. Gdr 19:51, 2004 Oct 19 (UTC)

Other problems:

- Kite does not need to be convex.

- Does one require a tangential to be convex? (I would say no.)

- Does one require cyclics to be simple? (I would say no. But then one might want so say that cyclic and simple implies convex.)

129.199.98.65 (talk) 14:35, 21 November 2008 (UTC)

A tangential quadrilateral would have to be convex. If you try to draw a concave quadrilateral around a circle, you will get stuck: at the concavity, the path would have to move away from the circle.

As such, only the convex case of a kite is tangential. Though you could, if you wanted, make "kite" a subtype of "simple", and then "dart" and "convex kite" subtypes of this and "concave" and "tangential" respectively.

Looking at it, complex quadrilaterals can be cyclic. ISTM they violate the theorem that opposite angles add to 180°. I'll try and get my head around it.... -- Smjg (talk) 21:55, 27 March 2010 (UTC)

Congruent or equal

Whilst I am completely happy to use the term "congruent", I wonder if the general reader (for whom this article is intended) would find the word "equal" simpler to understand? Dbfirs 10:52, 2 January 2009 (UTC)

Schools in the US seem to prefer to use "congruent", but "equal" (or "isos") has been used for at least 2000 years since Euclid. Dbfirs 09:30, 13 January 2009 (UTC)

Any idea why the US schools seem to prefer "congruent"? Are they being nonconformists, malcontents, provocateurs? Don't they know any better than to continue to follow the same usage all the way back to antiquity? Perhaps, just perhaps mind you, they found something lacking in the term and decided sometime during those 2000 years to develop a more appropriate, more accurate term. Nah! We'll just go with you're implication that US educators don't know any better. Do you know what else has been around for more than 2000 years? Bigotry. JackOL31 (talk) 05:15, 20 November 2009 (UTC)

But the term that they invented does not match the usage of "congruent" agreed by all mathematicians (possibly since 1578) in other geometric contexts. In the UK, we still teach elementary Euclidean geometry not some botched attempt to reform it! - yes, that bit is bigotry! I am told that not all American mathematicians agree with your rejection of "equal", but I am happy to compromise and simply report facts in a way that is clear to all. Dbfirs 07:49, 20 November 2009 (UTC)

I'd like to put in my 2 cents as an educator in the US. Numbers that are the same are said to be "equal" while objects that are the same are said to be "congruent." It gets a little confusing for students because you can say "Angle 1 is congruent to Angle 2" and say "the measure of Angle 1 is equal to the measure of angle 2." This is still consistent because the 'measures' are numbers and are thefore 'equal' while the angles themselves are objects and stand to be congruent. Andrewbressette (talk) 19:50, 16 December 2009 (UTC)

I think the distinction between equal numbers and congruent objects is exactly the same in the UK and throughout the world. Where we differ is that we define angle as an amount of turn (a number), so we use "equal". Angles as objects can be congruent if they are defined in terms of the set of points bounded by infinite rays, but we have a problem with representations of "equal" angles that are clearly not congruent as figures. Dbfirs 22:14, 27 March 2010 (UTC)

Angles are certainly geometrical entities. Maybe it's best to treat them not as sets of points, but as sets of rays having a common origin. The measure of an angle is then a measure of this set of rays, just as the length of a line segment is a measure of this set of points.

For plane angles, equal and congruent are the same. (You could debate how dihedral angles fit into the equation.) But solid angles can be equal without being congruent. -- Smjg (talk) 00:08, 30 March 2010 (UTC)

Good point about solid angles! Our article uses only the concept of measure of solid angle, allowing only equality. Should we extend it to mention the concept of congruence of solid angles? What would be a Hilbert-style definition of solid angle?
The half-rays have to be infinite for equal plane angles to be congruent. Dbfirs 17:24, 30 March 2010 (UTC)

... could we say that solid angles are congruent if they have the same measure and they project similar areas onto a sphere centred at the vertex? Dbfirs 17:41, 30 March 2010 (UTC)

Half-rays? A ray is, by definition, infinite in exactly one direction. Apparently "half-line" is a synonym for ray. What would a half-ray be?

As for solid angles ... if you mean similar in the geometric sense, then yes. Another way of describing it is that they project congruent figures onto equal-radius spheres centred at their respective vertices. -- Smjg (talk) 19:26, 1 April 2010 (UTC)

Yes, I did mean half-line, i.e. ray, and also geometrically similar, but perhaps your congruent on spheres of equal radius is clearer. Should this be added to solid angle? Dbfirs 22:59, 1 April 2010 (UTC)

but —Preceding unsigned comment added by 74.248.224.78 (talk) 20:44, 7 April 2010 (UTC)

Which geometry?

We seem to be unclear about which sections apply to Euclidean plane geometry, and which apply to any geometry, including the twisted geometry of crossed quadrilaterals. Should we leave the other geometries to the end? Dbfirs 07:57, 27 April 2010 (UTC)

Do the interior angles at the intersection of a complex quadrilateral really add up to 720 degrees?

The reference for the 720 figure states that 720 is the total of two exterior angles and two interior, not the angles at the intersection. Or am I missing something? --Cheesychipmunk (talk) 20:15, 14 December 2010 (UTC)

You should follow the reference how it is measured. --Octra Bond (talk) 08:48, 1 March 2011 (UTC)

_

A Venn Diagram with all the special types of quadrilaterals should be on the page as an image. I have a rough, crude drawing in MS paint, just to show the idea. A special note about the "kite" shape should be in the page, explaining how it's not exactly a proper mathematical name, although it could certainly be described as "a quadrilateral with two pairs of equal side lengths, and one pair of equal opposite angles" (or the likes). although not necessarily in the venn diagram image

http://img29.imageshack.us/img29/1620/quadrilateralvenndiagra.png — Preceding unsigned comment added by 69.231.70.215 (talk) 07:23, 26 July 2011 (UTC)

Rhomboid

Isn't the British version of "Rhombus" "Rhomboid"? — Preceding unsigned comment added by 111.125.108.144 (talk) 07:49, 6 June 2012 (UTC)

No. Dbfirs 07:59, 6 June 2012 (UTC)

Caveat needed?

The section Other metric relations says

The shape of a convex quadrilateral is fully determined by the lengths of its sides and one diagonal.

Does this need to be qualified to something like this?

The shape of a convex quadrilateral is fully determined by the lengths of its sides in sequence and of one diagonal between two specified vertices.

Duoduoduo (talk) 15:00, 28 December 2012 (UTC)

Yes, good point. I tried to think of a better way to express the statement without mentioning "specified vertices", but I can't improve on your version. Dbfirs 19:22, 28 December 2012 (UTC)

Rhombus error

Under "Convex Quadrilaterals - Parallelograms" the following paragraph regarding the rhombus has a mistaken equivalent condition in it. It reads: "Rhombus or rhomb: all four sides are of equal length. Equivalent conditions are that opposite sides are parallel and opposite angles are equal, or that the diagonals perpendicularly bisect each other."

The first equivalent condition given (that opposite sides are parallel and opposite angles are equal) is true of any parallelogram even if it doesn't happen to be a rhombus. The last equivalent condition (that diagonals are perpendicular bisectors of each other) is particular to only the rhombus, so it works. I'm not sure what the original writer may have had in mind when writing the first equivalent condition, so I'm in no position to correct it; but I did remove it since it fails as such. --geometry teacher (216.147.238.178 in March 2013)

Yes, you were correct to remove this. The error was introduced in an edit of 21:38, June 5, 2009 by Tropylium who, I think, is more of a linguist than a mathematician, and we (other editors) should have spotted it much earlier! I don't know how I missed it! Dbfirs 08:26, 15 March 2013 (UTC)

Trapezoid dual

Does anyone know of any special name for a quadrilateral whose dual is a trapezoid?? Wikipedia appears to reveal nothing about this kind. Georgia guy (talk) 18:49, 18 March 2013 (UTC)