1. UNIT 9: GEOMETRY – 6 TH GRADE LESSON 5: AREA OF A CIRCLE

2. CORNELL NOTES HEADER Topic: Geometry 6 th Grade (Unit 9 pg. 5) E. Q.: How does the area of a circle relate to the area of a square or rectangle? Name: _____________________________ Date: _____________________________ Class: _____________________________ Chp. 10 Lesson 4 –Area of a Circle

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 r 2 where π = pi r = radius r 2 = 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 = πr 2 A = (3.14)(3) 2 A = (3.14)(9) A = 28.26 cm 2 *Estimate to check: 3 x 9 = 27

5. AREA OF A CIRCLE EXAMPLE Ex. Find the area of the circle. 20 cm A = πr 2 A = (3.14)(10) 2 ** divide the diameter by 2 to get the radius A = (3.14)(100) A = 314 cm 2 *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. A rec = lw A halfcir = ½ π r 2 A = (19)(12) A = ½ (3.14)(6) 2 A = 228 ft 2 A = 56.52 ft 2 Estimate: 20(12) = 240 Estimate: ½(3)(36) = 54 A freethrow = A rec + A halfcir = 228 + 56.52 = 284.52 ft 2 or 285 ft 2 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? A 8in = πr 2 A 16in = π r 2 A = 3.14(4) 2 A = 3.14(8) 2 A = 50.24 in 2 A = 200.96 in 2 200.96 = 4 50.24 The area of a 16 inch pizza is four times the area of an eight inch pizza.