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Chapter 14

Chapter 14. Regular Polygons and the Circle. Regular Polygons. A regular polygon is a convex polygon that is both equilateral and equiangular. Regular hexagon. Regular pentagon. Equilateral triangle. square. Theorem : Every regular polygon is cyclic.

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Chapter 14

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  1. Chapter 14 Regular Polygons and the Circle

  2. Regular Polygons • A regular polygon is a convex polygon that is both equilateral and equiangular Regular hexagon Regular pentagon Equilateral triangle square

  3. Theorem : Every regular polygon is cyclic • Given: ABCDE is a regular polygon • Prove: ABCDE is cyclic Draw the circle that contains points A, B, and C, and let its center be called O. Draw OA, OB, OC, and OD. OB=OC (all radii of a circle are equal) Therefore, 1= 2 (if 2 sides of a triangle are equal, the are equal, the angles opposite them are equal) Because ABC= BCD (a regular polygon is equiangular), 3= 4 (subtraction). Also, AB = CD (a regular polygon is equilateral), and so ∆OBA =∆OCD (SAS) Therefore, OD= OA And because OA is the radius of the circle, it follows that OD also is a radius.

  4. Vocab… • A polygon whose vertices lie on a circle is inscribed in the circle and the circle is circumscribed about the polygon • The center of a regular polygon is the center of its circumscribed circle • The word radius, as for circles, can refer either to a line segment or to a distance: a line segment that connects the center of a regular polygon to a vertex or the distance between the center and that vertex. • A central angle of a regular polygon is an angle formed by radii drawn to two consecutive vertices • An apothem of a regular polygon is a perpendicular line segment from its center to one of its sides.

  5. The perimeter of a regular polygon • The perimeter of a regular polygon can be found by multiplying the length of a side by the number of sides. • Doing so gives us the equation: p= ns where p is the perimeter of the polygon n is the number of sides s is the length of one side

  6. Theorem: • The perimeter of a regular polygon having n sides is 2Nr in which N= n(sin180/n) and r is its radius • This formula shows that the perimeter of a regular polygon is proportional to tis radius

  7. The area of a regular polygon • The area of a regular polygon having n sides is Mr2 in which M= n(sin180/n)(cos180/n) and r is its radius

  8. Vocab… • The polygons are inscribed in the circles; so there is a limit to how large the perimeters can get. This limit is the length, or circumference, of the circle • The circumference of a circle is the limit of the perimeters of the inscribed regular polygons

  9. Theorem • If the radius of a circle is r, its circumference is 2r because the diameter of a circle is twice its radius, it follows from the equations c=2r and d=2r that c=d • Corollary: If the diameter of a circle is d, its circumference is d

  10. Area of a circle • The area of a circle is the limit of the areas of the inscribed regular polygons • By using the area formula, it is clear that as the number of sides (n) increases, M gets closer and closer to . Hence, the areas of the polygons are getting closer and closer to the number r2 • Theorem: If the radius of a circle is r, its area is r2

  11. Sectors and Arcs • A sector of a circle is a region bounded by an arc of the circle and the two radii to the endpoints of the arc. If the circle had a radius of 12 inches, and was cut into 8 equal pieces, the area of each piece is 1/8(12)2 =18 57 square inches Because the border of the circle (360°) also is divided into eight congruent pieces, each sector has an arc (and central angle) with a measure of 1/8(360°) =45° The lengths of these eight equal arcs add up to give the circumference of the circle; so each arc has length 1/8(2)(12)= 3 9 inches

  12. Because the arc of every circle has a measure of 360°, the area of the sector must be m/360 times the area of the circle, or m/360r2 . The length of its arc is m/360 times the circumference of the circle, or m/360(2)r. The idea is simple: If a sector is a certain fraction of a circle, then its area is the same fraction of the circle’s area. If an arc is a certain fraction of a circle, then its length is the same fraction of the circle’s circumference.

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