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A quadrilateral is a polygon with four sides and four vertices. The following figure has four sides and four vertices but sides JK and KL are in a straight line. So, can we consider this figure as a quadrilateral? or is it a triangle? but why?

enter image description here

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The polygon has three sides and vertices, so it is a triangle. Think about the shape not as having two sides that are in a straight line, but as a normal triangle simply with an extra point labeled on one of the sides.

Consider that if adding an extra point changed the shape, I could make this shape any $n$-gon I want.

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  • $\begingroup$ @H Huang Now only I got it. Thanks. $\endgroup$
    – Sara
    May 9 '20 at 18:44

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