Posted by polygon on Wednesday, March 28, 2012 at 1:43am.
at each vertex, interior + exterior = 180 degrees.
If exterior = x,
x+(8x+18) = 180
9x = 162
x = 18
he exterior angle of a regular n-gon is 360/n, so
18 = 360/n
n = 20
Interior angle = 180 - (exterior angle) = 180 -360/N
Exterior angle = 360/N
180 - (360/N) = [8*360/N] +18
9*360/N = 162
N = 20 sides
It is called an icosagon
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