Assume each side is a straight line segment.
With 3 sides you can make 1 triangle with a sum of interior angles = 180°.
With 4 sides you can make 2 triangles with a sum of interior angles = 2 x 180° = 360°.
With 5 sides you can make 3 triangles with a sum of interior angles = 3 x 180° = 540°.
The exterior angles of a 15 sided shape add up to 360 degrees
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360
The exterior angles of a 15 sided shape add up to 360 degrees
The interior angles of a 24 sided shape add up to 3960 degrees
The internal angles of an n-sided shape add to (n-2)*180 degrees. So, for an 8-sided shape, they would add to 6*180 = 1080 degrees.
The total sum of angles of an n-sided shape is (n-2)x180o Hence, the angles in a five sided shape add up to (5-2)x180o = 3 x 180o = 540o
The interior angles of a 9 sided nonagon add up to 1260 degrees and its exterior angles add up to 360 degrees
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540
The 8 interior angles of an 8 sided octagon add up to 1080 degrees
The interior angles of a 15 sided polygon sum to 2340 degrees