Points that are quadrilaterals

Question

When are 4 points in a plane considered quadrilaterals? Also, could you give me examples of 4 points that are not considered quadrilaterals? Thank you in advance!

A quadrilateral is a four-sided shape, not a collection of points. What would you expect to find here that the first paragraphs of the wikipedia article wouldn't give you? — Robert Mastragostino, Feb 19 '13 at 04:51
Did you mean when do 4 points form the vertices of a quadrilatetal? In that case you just want two make sure no three points are co-linear. So a bad example would be choosing 4 points on the same line. They can not be turned into a quadrilatetal. — Sean Ballentine, Feb 19 '13 at 05:11

score 1 · Accepted Answer · edited Feb 19 '13 at 12:55

1

The definition varies depending on the user and the application, but the biggest challenges are 1) three points in a line, so an angle is $\pi$: $(0,0), (0,1), (0,2), (1,1)$, 2) a crossing quadrilateral $(0,0), (2,0), (0,2), (2,2)$, 3) a concave quadrilateral: $(0,0), (10,0), (10,10), (9,1)$

$\hspace{1.5cm}$ enter image description here

edited Feb 19 '13 at 12:55

robjohn

345,667

answered Feb 19 '13 at 05:11

Ross Millikan

374,822

I hope you don't mind the addition of the images. Feel free to delete if you wish. – robjohn Feb 19 '13 at 12:55
@robjohn: thanks. I think they help. – Ross Millikan Feb 19 '13 at 14:03

Points that are quadrilaterals

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