## How Do You Make A Trapezoid Right?

I haven’t got done listing kinds of trapezoids, of course. Arguably I’d never be able to finish, since, after all, couldn’t any possible length of the two bases — the parallel lines — and of different lengths of the diagonal legs be imagined? Well, perhaps, although a lot of those kinds are going to look the same. An isoceles trapezoid where the long base is 10 and the short base 8 looks a lot like one where the long base is 11 and the short base 7.5, at least if the bases are the same distance apart. But there are more cases imaginable.

One of the common assumptions made in drawing a generic trapezoid is to put the longer base on bottom, and the shorter on top. There’s no reason we have to assume that, though; the longer base can be on top just as easily. All it offends is the idea that the trapezoid might look like a rectangle rendered in one-point perspective.

So far another little assumption slipped into all these diagrams has been making the parallel bases themselves also horizontal. If we’re just sketching something on a piece of paper we may as well lay things down that way — we have to put the baselines somewhere, after all — but it’s not required that it be. We can place the parallel lines in any direction, and if it isn’t obvious what we need that for, we probably haven’t done much work on creatively tiling floors or sketching out grid patterns while taking notes.

Another class of trapezoids, and an important one, is called the right trapezoid. In this, one of the legs intersects the bases (both of them) at right angles, making the trapezoid look like a profile of the Citicorp Center building in Manhattan. Apart from having a name that reminds one of right triangles — and if one extended the legs, we’d get right triangles, from either base to where the legs come together — it turns out to be useful as a way into calculus.

We can compose a more extreme case of a right trapezoid: why not make

*both*legs intersect the bases at right angles? More commonly we’d call this a rectangle, and I imagine that labelling this a trapezoid would get a protest and maybe a point marked off an exam, at least until one protested to the grader. And even that might not settle the point. The extremely mildly shocking fact is there isn’t universal agreement on this point. The definitions of things like “trapezoid” are not fixed and unalterable. It’s convenient to have a name for this four-sided figure with two parallel bases. But we already have names for four-sided figures with two sets of parallel lines in them: they’re rectangles or at least parallelograms. Do we want the word “trapezoid” to also cover what’s already well-described by “parallelogram” and “rectangle”? (And, for that matter, “rhombus” and “square”?)

Reasonable people could disagree. For that matter, people might want to have trapezoids include rectangles and parallelograms in one context and not to have it be inclusive in another context. Mathematical language is, like any human language, a way of communicating ideas, and the exact definitions of terms has to depend on what’s useful to talk about. Let me show some context where it makes sense to be so inclusive.

One of the big questions in calculus is “how much area is there underneath this curve?” (where we mean under some curve, and above some bottom line, and between some left and right limits). This gets to be known as the integral of a function representing the curve. It turns out that the area underneath the curve can be estimated by breaking up the left-to-right limits into a couple of smaller widths, and approximating the curve with a set of right trapezoids. The vertical lines are the bases of the right trapezoids, and the horizontal line at the bottom is the leg perpendicular to the bases. The other leg connects the points where the curve intersects the vertical lines.

The sum total of the areas of these trapezoids is not going to be exactly equal to the area underneath the curve, except in a few freak cases. However, if the difference is smaller than some tolerable margin of error, that’s as good as being right. This sum of the areas of these trapezoids that approximate our original curve is an estimate of the area under the actual curve. Using trapezoids this way is known imaginatively as the use of the Trapezoid Rule. An we can use these trapezoids, giving us approximations to the area under a curve, as a tool to finding the exact areas, which is about the point in a calculus course where people who’ve just got the hang of derivatives realize they don’t understand a word of what’s going on. For many, this confusion will pass.

But it’s also a context in which we probably would want “trapezoids” to include “rectangles”: it’s possible for one or more of the trapezoids formed by this breaking up of the left-to-right range and the approximation of the shape with trapezoids to actually be a rectangle. It would be incredibly tedious to talk about approximating the shape with “trapezoids and possibly rectangles or squares”, though, so we just say it’s “trapezoids” and either implicitly add that “and possibly rectangles or squares” or else just take trapezoids as including their related shapes.

## fluffy (@fluffy) 12:36 am

onWednesday, 4 April, 2012 Permalink |But mathematically-speaking, the trapezoid method can also be considered as taking the mean of the curve height and using that to compute the area beneath the rectangle that the curve averages out.

You should be using Simpson’s Rule, anyway.

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## Joseph Nebus 3:14 am

onWednesday, 4 April, 2012 Permalink |I believe I know what you mean, although, are you certain you want to say the trapezoid area is the same as that of the rectangle with height the mean of the curve height?

Simpson’s rule is fine and dandy, but I don’t know anyone who can remember it without looking it up. The trapezoid rule has that advantage if you’re stranded on a desert island and need to do some computations.

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## Drawing A Trapezoid’s Picture | nebusresearch 2:24 am

onThursday, 5 April, 2012 Permalink |[…] Responses Joseph Nebus on How Do You Make A Trapezoid…fluffy (@fluffy) on How Do You Make A Trapezoid…How Do You Make A Tr… on Setting Out […]

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## How Many Trapezoids Can You Draw? | nebusresearch 2:28 am

onWednesday, 11 April, 2012 Permalink |[…] In a pair of earlier essays I talked about the problem of drawing differently-shaped trapezoids so as to not overlook figures that might be trapezoids just because the intuition focuses on one shape over […]

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## Donna 3:52 am

onWednesday, 11 April, 2012 Permalink |You can rederive Simpson’s Rule though. :)

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## Joseph Nebus 1:49 am

onThursday, 12 April, 2012 Permalink |Ah, that is an excellent theory, although I wouldn’t want to put it to the test. Gaussian Integration,

thatI’m sure I could rederive.LikeLike

## Everything I Know About Trapezoids | nebusresearch 12:28 am

onTuesday, 17 April, 2012 Permalink |[…] Responses How Many Trapezoids … on How Many Trapezoids Can You…Joseph Nebus on How Do You Make A Trapezoid…Joseph Nebus on Early April’s Math …Donna on How Do You Make A […]

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## My June 2013 Statistics | nebusresearch 1:48 pm

onMonday, 1 July, 2013 Permalink |[…] kinds of trapezoids there are — with such catchy titles as How Many Trapezoids I Can Draw, or How Do You Make A Trapezoid Right?, or Setting Out To Trap A Zoid, which should be recognized as a Dave Barry […]

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## nebusresearch | 14,000 7:00 pm

onSaturday, 8 March, 2014 Permalink |[…] by the way; some of the other articles which went into that (like Setting Out to Trap A Zoid and How Do You Make A Trapezoid Right) are also in the top five. All that’s based around working out how to work out a formula for […]

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