To find the measure of the add of all interior angles of a polygon, there is an easy formula:
(n-2)x180, n=number of vertices
Quadrilateral:
(4-2)x180=360 degrees, 360/4=90 degrees each angle
Pentagon:
(5-2)x180=540 degrees, 540/5= 108 degrees each angle
Dodecagon:
(12-2)x180=1800 degrees, 1800/12=150 degrees each angle
