There are a total of 10 interior angles in the decagon. Now for all polygons the sum of all the exterior angles is 360 degrees. There are 10 exterior angles in the decagon and, since it is regular, each of them = 360/10 = 36 degrees. Each interior angle therefore measures 180 - 36 degrees and the sum of the interior angles is this value times ten.
There are a total of 10 interior angles in the decagon. Now for all polygons the sum of all the exterior angles is 360 degrees. There are 10 exterior angles in the decagon and, since it is regular, each of them = 360/10 = 36 degrees. Each interior angle therefore measures 180 - 36 degrees and the sum of the interior angles is this value times ten.
