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Circumference and Areas of Circles Lesson 11.5 Geometry Honors Objective: Know and use the formulas for the Circumference and Areas of Circles

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Lesson Focus In this lesson, the formulas for circumference and area of a circle are stated. These formulas can be understood by using facts that have been developed about areas of regular polygons.

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Summary Basic Terms Circumference The perimeter of or distance around a circle. Area of a Circle The area of a disk (the region inside a circle) is πr 2 when the circle has radius r. π An irrational number that is the ratio of the circumference of a circle to its diameter.

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Area of a Circle Area, A = πr 2

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Circumference Circumference, C = 2πr = πd

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Pi Leave answers in π form unless an approximation is specified. The exact circumference or exact area of a circle requires that π be approximated. π is an irrational number is not the ration of two integers, as the quotient 22/7 suggests. In the expression C/d, d is rational, C is not, and vice versa.

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Practice 1.What is the ratio of the circumference to the diameter for each circle? 2.A circle has a radius of 15. What is the diameter, circumference, and area of the circle? 3.A circle has a diameter of 8. What is the radius, circumference, and area of the circle? 4.A circle has a circumference of 26π. What is the radius, diameter, and area of the circle? 5.A circle has an area of 100π. What is the radius, diameter, and circumference of the circle?

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Written Exercises Problem Set 11.5, p.448: # 2 – 16 (even) Handout: 11-5

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