Concave polygons are polygons for which a line segment joining any two points in the interior
lies completely within the figure
The word interior is important. You cannot choose one point inside and one point outside the figure
The following figure is concave:
Segment AB does not entirely lie within the polygon. That is why the polygon is concave
Notice that it is quite possible to find other segments that will lie inside the figure, such as segment FE.
However, if you can find at least one
segment that does not lie within the figure, the figure is concave
The following figure is also concave
It is easy to construct a concave figure if the figure has at least 4 sides
Just make sure that one interior angle is bigger than 180 degrees.
In other words, an interior angle should be a reflex angle
Why am I saying at least 4 sides? It is possible to make a concave triangle?
The answer is no!
Since the sum of the interior angles in any triangle must add up to 180 degrees, no interior angles can be more than 180. It is impossible!
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