What is the sum of the interior angle measures of a regular octagon

You would first need to find the exterior angle of the octagon first in order to find the interior angle of the octagon. Since all the exterior angles of any polygon is 360, you would divide 8 from 360 which is 45

Exterior Angle=45

Then, you would subtract 45 from 180, since the exterior angle + the interior angle is equal to 180 degrees.

180-45=135

Exterior Angle=135

Take the amount of angles and divide 360 by said amount and your answer is there.