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# Lesson 8-1

Lesson 8-1. Angles of Polygons. Objectives. Find the sum of the measures of the interior angles of a polygon Sum of Interior angles = (n-2) • 180 One Interior angle = (n-2) • 180 / n Find the sum of the measures of the exterior angles of a polygon Sum of Exterior angles = 360

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## Lesson 8-1

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1. Lesson 8-1 Angles of Polygons

2. Objectives • Find the sum of the measures of the interior angles of a polygon • Sum of Interior angles = (n-2) • 180 • One Interior angle = (n-2) • 180 / n • Find the sum of the measures of the exterior angles of a polygon • Sum of Exterior angles = 360 • One Exterior angle = 360/n • Exterior angle + Interior angle = 180

3. Vocabulary • Diagonal – a segment that connects any two nonconsecutive vertices in a polygon.

4. 3 2 4 1 5 6 8 7 Angles in a Polygon Octagon n = 8 8 triangles @ 180° - 360° (center angles) = (8-2) • 180 = 1080 Sum of Interior angles = (n-2) • 180

5. Angles in a Polygon Sum of Interior Angles: (n – 2)* 180 where n is number of sides so each interior angle is (n – 2) * 180 n Octagon n = 8 Interior Angle Sum of Exterior Angles: 360 so each exterior angle is 360 n Interior Angle + Exterior Angle = 180 Exterior Angle Octagon Sum of Exterior Angles: 360 Sum of Interior Angles: 1080 One Interior Angle: 135 One Exterior Angle: 45

6. Polygons

7. Example 1-1a ARCHITECTUREA mall is designed so that five walkways meet at a food court that is in the shape of a regular pentagon. Find the sum of measures of the interior angles of the pentagon. Since a pentagon is a convex polygon, we can use the Angle Sum Theorem. Interior Angle Sum Theorem Simplify. Answer: The sum of the measures of the angles is 540.

8. Example 1-2a The measure of an interior angle of a regular polygon is 135. Find the number of sides in the polygon. Use the Interior Angle Sum Theorem to write an equation to solve for n, the number of sides. Interior Angle Sum Theorem Distributive Property Subtract 135n from each side. Add 360 to each side. Divide each side by 45. Answer: The polygon has 8 sides.

9. SHORT CUT!! The measure of an interior angle of a regular polygon is 135. Find the number of sides in the polygon. Exterior angle = 180 – Interior angle = 45 360 360 n = --------- = ------- = 8 Ext  45

10. Example 1-3a Find the measure of each interior angle.

11. Example 1-4a Find the measures of an exterior angle and an interior angle of convex regular nonagon ABCDEFGHJ. At each vertex, extend a side to form one exterior angle. Answer: Measure of each exterior angle is 40. Since each exterior angle and its corresponding interior angle form a linear pair, the measure of the interior angle is 180 – 40 or 140.

12. Polygon Hierarchy Polygons Quadrilaterals Parallelograms Kites Trapezoids IsoscelesTrapezoids Rectangles Rhombi Squares

13. Quadrilateral Characteristics Summary Convex Quadrilaterals 4 sided polygon 4 interior angles sum to 360 4 exterior angles sum to 360 Parallelograms Trapezoids Bases Parallel Legs are not Parallel Leg angles are supplementary Median is parallel to basesMedian = ½ (base + base) Opposite sides parallel and congruent Opposite angles congruent Consecutive angles supplementary Diagonals bisect each other Rectangles Rhombi IsoscelesTrapezoids All sides congruent Diagonals perpendicular Diagonals bisect opposite angles Angles all 90° Diagonals congruent Legs are congruent Base angle pairs congruent Diagonals are congruent Squares Diagonals divide into 4 congruent triangles

14. Homework • Homework: • pg 407-408; 13-23 (omit 17,18), 27-32, 35-38

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