In general: the sum of the interior angles in a n sided polygon is
A regular polygon is a polygon where the sides are all of equal length and the angles are all congruent (or of equal measure).
name of polygon  n 
degrees of the interior measure of each angle 
360 degrees divided by # in Column 2 
equilateral triangle  3 
60  360 / 3 = 120 
square  4 
90  360/4= 90 
regular pentagon  5 
3*180/5= 108 
360/5= 72 
regular hexagon  6 
4*180/6=120 
360/6= 60 
regular heptagon  7 
5*180/7 
360/7 
regular octagon  8 
6*180/8=135 
360/8 = 45 
regular dodecagon 
12 
10*180/12=1800/12=150

360/12=30 
* 
Id 
R120 
R240 
V 
G=R1 
H=R2 
Id 
I 

R120 
R240 
R1 

R240 
I 
R2 

V 
R2 
R1 
I 
R120 

G=R1 
I 

H=R2 
I 