icosikaienneagonal

Icosikaienneagonal refers to a polygon with 21 sides. The term is derived from Greek: "eikosi" meaning twenty, "kai" meaning and, and "ennea" meaning nine, combined with "gonia" meaning angle or corner. Thus, icosikaienneagonal literally translates to "twenty and nine-angled".

A polygon with 21 sides is also known as a icosikaienneagon. In Euclidean geometry, an icosikaienneagon has 21 vertices and 21 edges. The sum of the interior angles of any icosikaienneagon is given by the formula (n-2) * 180 degrees, where n is the number of sides. For a 21-sided polygon, this sum is (21-2) * 180 = 19 * 180 = 3420 degrees.

A regular icosikaienneagon has all sides equal in length and all interior angles equal. Each interior angle of a regular icosikaienneagon measures 3420 / 21 degrees, which simplifies to approximately 162.86 degrees. The exterior angles of a regular icosikaienneagon each measure 360 / 21 degrees, approximately 17.14 degrees.

The construction of a regular icosikaienneagon is possible using a compass and straightedge, as 21 is a product of distinct Fermat primes (3 and 7). However, it is a complex construction due to the number of sides.