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Unit 9: Geometry – 6 th Grade

Unit 9: Geometry – 6 th Grade. Lesson 5: area of a Circle. Cornell Notes Header. Vocabulary Review. Pi - The ratio of a circle’s circumference to its diameter. To approximate pi, use 22/7 or 3.14. When the diameter or radius is a multiple of 7, it will be easier to use 22/7 for π.

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Unit 9: Geometry – 6 th Grade

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  1. Unit 9: Geometry – 6th Grade Lesson 5: area of a Circle

  2. Cornell Notes Header

  3. Vocabulary Review • Pi - • The ratio of a circle’s circumference to its diameter. To approximate pi, use 22/7 or 3.14. When the diameter or radius is a multiple of 7, it will be easier to use 22/7 for π. • diameter – • The distance across the circle through its center, d. • circumference – • The distance around a circle, C. • radius – • the distance from the center to any point on the circle, r.

  4. area of a circle • Formula for Area of a Circle: A = π r2 • where π = pi r = radius r2 = r r • Ex. You are making a design for a circular button. Your design fits on a circle with a radius of 3 cm. How much area will be covered by your design? • 3 cm • A = πr2 • A = (3.14)(3)2 • A = (3.14)(9) • A = 28.26 cm2 • *Estimate to check: 3 x 9 = 27

  5. Area of a circle Example • Ex. Find the area of the circle. • 20 cm • A = πr2 • A = (3.14)(10)2 ** divide the diameter by 2 to get the radius • A = (3.14)(100) • A = 314 cm2 • *Estimate to check: 3 x 100 = 300

  6. Finding the area of combined figures • Find the area of each shape in the figure and then add the areas to find the total area. • Ex. Find the area of the free throw area to the nearest square foot. • Arec = lwAhalfcir = ½ π r2 • A = (19)(12) A = ½ (3.14)(6)2 • A = 228 ft2 A = 56.52 ft2 • Estimate: 20(12) = 240 Estimate: ½(3)(36) = 54 • Afreethrow= Arec + Ahalfcir • = 228 + 56.52 = 284.52 ft2 or 285 ft2 • Estimate: 240 + 54 = 294 6 ft 12 ft 19 ft

  7. Group Problem • How many times as great as the area of an 8 inch pizza is the area of a 16 inch pizza? • A8in = πr2 A16in = π r2 • A = 3.14(4)2 A = 3.14(8)2 • A = 50.24 in2 A = 200.96 in2 • 200.96 = 4 • 50.24 • The area of a 16 inch pizza is four times the area of an eight inch pizza.

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