1 objectives define polygon, concave / convex polygon, and regular polygon find the sum of the...
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1
Objectives
• Define polygon, concave / convex polygon, and regular polygon
• Find the sum of the measures of interior angles of a polygon
• Find the sum of the measures of exterior angles of a polygon
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Definition of polygon• A polygon is a closed plane figure formed
by 3 or more sides that are line segments;– the segments only intersect at endpoints– no adjacent sides are collinear
• Polygons are named using letters of consecutive vertices
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Concave and Convex Polygons
• A convex polygon has no diagonal with points outside the polygon
• A concave polygon has at least one diagonal with points outside the polygon
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Regular Polygon Definition
• An equilateral polygon has all sides congruent
• An equiangular polygon has all angles congruent
• A regular polygon is both equilateral and equiangular
Note: A regular polygon is always convex
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Sum of Interior Angles in Polygons
Convex Polygon # of Sides
# of Triangles from 1 Vertex
Sum of Interior Angle Measures
Triangle 3 1 1* 180 = 180
Quadrilateral 4 2 2* 180 = 360
Pentagon 5 3 3* 180 = 540
Hexagon 6 4 4* 180 = 720
Heptagon 7 5 5* 180 = 900
Octagon 8 6 6* 180 = 1080
n-gon n n – 2 (n – 2) * 180
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Example: Sum of Interior Angles
Find m∠ X
Solution: The sum of the measures of the interior angles for a quadrilateral is (4 – 2) * 180 = 360
The marks in the illustration indicate that
m∠X = m∠Y. So the sum of all four interior angles is m∠X + m∠X + 100 + 90 = 3602 m∠X + 190 = 3602 m∠X = 170 m∠X = 85
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Polygon Exterior Angle Sum Theorem
• The sum of the measures of the exterior angles of a polygon, one at each vertex is 360.
m∠1 + m∠2 + m∠3 + m∠4 + m∠5 = 360
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Example: Exterior Angle Sum
What is the measure of an interior angle of a regular octagon?
Solution:
8 * exterior angle = 360 (Ext. Angle Sum)
exterior angle = 45
interior angle = 180 – exterior angle
interior angle = 180 – 45 = 135
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