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Topic: Classification of quadrilaterals
Replies: 17   Last Post: Jan 11, 1995 6:43 AM

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Chenteh Kenneth Fan

Posts: 14
Registered: 12/6/04
Re: Classification of quadrilaterals
Posted: Dec 21, 1994 3:01 PM
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In article <9412211609.AA06198@broccoli.princeton.edu>, conway@math.Princeton.EDU (John Conway) writes:
|> I'm not quite sure why you (ckfan) WANT to include the general
|> trapezoid in your classification!
|>
|> I don't know of any interesting theorems on the trapezoid that
|> don't work for more general quadrilaterals, and it's always rather
|> puzzled me why anyone ever thought it worth while to give this
|> particular kind of quadrilateral its own special name.
|>
|> I now think I know the reason, and am not too impressed by it.
|> Proclus (who's the guy responsible) says that the area is the
|> mean of the lengths of the two parallel sides times the distance
|> between them, and I think this was the point - the trapezoid is
|> the most general quadrilateral with a simple area formula. So
|> it has some practical point - you can compute the area of a
|> polygon by dividing it up into trapezoids.
|>
|> But there's not much theoretical point - this area formula
|> is trivially deducible from the one for a triangle by just
|> drawing a diagonal. Does anyone know of any more interesting
|> theorem about trapezoids?
|>
|> John Conway

Maybe you're right...because I don't even find myself particularly
interested in trying to think of "interesting theorems on the trapezoid
that don't work for more general quadrilaterals"! Perhaps I was guided
more by emotional reasons than mathematical ones!

But I at least offer these:

1. Which quadrilaterals tesselate the plane?

2. (i'm groping) Given a tesselation of the plane into congruent triangles,
which quadrilaterals can be formed from a finite union of these triangles?

3. (groping...groping...) Which quadrilaterally shaped billiard tables
allow for ball trajectories which touch fewer than four sides? (no friction...
no ball size...cross reference this weeks problem of the week!)

(Note: the answers to these all include the trapezoids...but the answers
do not include generic quadrilaterals.)

Ken








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