Question 1037922
Sum of the interior and exterior angle at the one vertex is 180 degrees for a straight angle.


The two angles are v and 3v.
{{{v+3v=180}}}
{{{4v=180}}}
{{{v=180/4}}}
{{{v=2*3*3*2*5/(2*2)}}}
{{{highlight(v=45)}}}--------the exterior angle.  The interior angle is 135 degrees.


 A route of thinking:  Starting with a triangle, regular polygon has 180 degrees, sum of the interior angles.   Four sides is added another 180 degrees.  5 sides, another 180 degrees.
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You can build a chart of this.

<pre>
n        Interior Degrees     Degrees at a vertex
3            180                 60
4            360                 90
5            540                108
6            720                120
7            900                128.57
8           1080                135
</pre>

There it is.   Eight sides regular polygon.