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

2. 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.

11. 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.

12. Area of a Circle Area, A = πr 2

13. Circumference Circumference, C = 2πr = πd

14. 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.

15. 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?

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