：Find x+y, if: 2x+3y=8 and 3x+5y=13
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Answer:
Types of polygon
Polygons can be regular or irregular. If the angles are all equal and all the sides are equal length it is a regular polygon.
Regular and irregular polygons
Interior angles of polygons
To find the sum of interior angles in a polygon divide the polygon into triangles.
Irregular pentagons
The sum of interior angles in a triangle is 180°. To find the sum of interior angles of a polygon, multiply the number of triangles in the polygon by 180°.
Example
Calculate the sum of interior angles in a pentagon.
A pentagon contains 3 triangles. The sum of the interior angles is:
180 * 3 = 540
The number of triangles in each polygon is two less than the number of sides.
The formula for calculating the sum of interior angles is:
(n - 2) * 180 (where n is the number of sides)
