SOLUTION: find the number of sides in a polygon when each interior angle is 18 degree

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Question 782796: find the number of sides in a polygon when each interior angle is 18 degree
Answer by KMST(5328) About Me  (Show Source):
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If all the highlight%28exterior%29 angles of a polygon measure 18%5Eo, the polygon has 360%5Eo%2F18%5Eo=highlight%2820%29sides.

The highlight%28interior%29 angles of a polygon, are the angles between adjacent sides highlight%28inside%29 the polygon.

The highlight%28exterior%29 angles of a polygon, are the angles between adjacent sides highlight%28outside%29 the polygon.

For example, in the polygon below,
all the exterior angles measure 72%5Eo, and
all the interior angles measure 108%5Eo.
(For any polygon, the measure of an interior angle and the measure of the adjacent exterior angle add up to 180%5Eo, because they are supplementary angles).



Each exterior angle is the angle that you deviate from your previous direction as you "turn the corner around a vertex. If you go one full turn around the whole polygon, the sum of all the (exterior) angles you turn is 360%5Eo.

IF all the exterior angles have the same measure, all the interior angles have the same measure (and vice versa).
IN THAT CASE, the measure of all those angles depends on the number of sides of the polygon. The measure of one exterior angle times the number of sides is the sum of all the exterior angles and equals 360%5Eo.
If n= number of sides and E is the measure each exterior angle, then
n%2AE=360%5Eo <--> n=360%5Eo%2FE.
{Also, n%2AE=360%5Eo <--> E=360%5Eo%2Fn).
The polygon with the least sides, would have 3 sides, with exterior angles measuring 120%5Eo and interior angles measuring 60%5Eo. With more sides the exterior angles would be smaller and the interior angles would be larger. The exterior angles could measure 18%5Eo and less, but the inerior angles cannot measure less than 60%5Eo.