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According to the definition of Polygon, If a Poly-line's first and last points are connected then it is called Polygon. See the image below. I have P1, .... P5 Polyline. If I draw a line from P5 to P1 then it will be called polygon or not? I am confused here.

enter image description here

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Being confused is perfectly reasonable here! Part of the problem is that there are several different definitions of polygon; to answer the question of whether your shape is a polygon or not, you'll have to be much more precise in the definition you're using. –  Steven Stadnicki Aug 13 '12 at 6:32
    
@StevenStadnicki, I am talking about this en.wikipedia.org/wiki/Polygon –  user960567 Aug 13 '12 at 6:50
    
Perhaps this would be relevant : en.wikipedia.org/wiki/Digon –  Patrick Da Silva Aug 13 '12 at 6:56
    
@PatrickDaSilva, It's mean the above shape can be called a polygon? –  user960567 Aug 13 '12 at 7:03
    
@user960567 : It means that some people would call that a polygon. Personally I don't think it's important to make a distinction, but if for you it is then some people agree that it can be considered a polygon. I have no idea if it's a good idea or not to do so, but I usually refer to polygons as non-degenerate ones. –  Patrick Da Silva Aug 13 '12 at 8:32

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I think conventionally you would expect the area of a polygon to be non-zero.

Generally your lecturer/teacher should specify in lectures the exact definition of things, and you should use that to answer his/her questions. If you find loopholes you should ask them about it.

According to the link for Wikipedia you supplied:

In geometry a polygon is a flat shape consisting of straight lines that are joined to form a closed chain or circuit.

Digging a little deeper you can find this at http://en.wikipedia.org/wiki/Closed_polygonal_chain:

In some cases it is important to draw a distinction between a polygonal area and a polygonal chain.

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