sum_of_interior_angles = (number_of_sides - 2) x 180°
→ if number_of_sides is 21: sum = (21 - 2) x 180° = 19 x 180° = 3420°
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The sum of the interior angles of any regular polygon of n sides is equal to 180(n - 2) degrees. 3420 degrees
The number of triangles formed by the diagonals of a 21 sided polygon is C(21,3) = 21x20x19/3x2x1 = 1,330.
No matter how many sides it has, the exterior angles of a regular polygon add up to 360 degrees. So 360/17.14 = 21 sides.
It will have 21 sides or a 21-gon
a decadogon