9-8 Circles and Circumference Warm Up Warm Up Lesson Presentation Lesson Presentation Problem of the Day Problem of the Day Lesson Quizzes Lesson Quizzes

9-8 Circles and Circumference Warm Up The length and width of a rectangle are each multiplied by 5. Find how the perimeter and area of the rectangle change. The perimeter is multiplied by 5, and the area is multiplied by 25.

9-8 Circles and Circumference Problem of the Day When using a calculator to find the width of a rectangle whose length one knew, a student accidentally multiplied by 20 when she should have divided by 20. The answer displayed was 520. What is the correct width? 1.3

9-8 Circles and Circumference Learn to identify the parts of a circle and to find the circumference of a circle.

9-8 Circles and Circumference Vocabulary circle center chord diameter radius (radii) ‏ circumference pi

9-8 Circles and Circumference A circle is the set of all points in a plane that are the same distance from a given point, called the center. Center

9-8 Circles and Circumference Radius (plural: radii). A line segment with one endpoint at the center of the circle and the other endpoint on the circle. Center Radius

9-8 Circles and Circumference A diameter is a chord that passes through the center of the circle. Diameters are the longest chords in a circle. Center Radius Diameter

9-8 Circles and Circumference A chord is a line segment that has both endpoints on the circle. Center Radius Diameter Chord

9-8 Circles and Circumference Additional Example 1: Naming Parts of a Circle Name the circle, a diameter, and three radii. N The circle is circle Z.LM is a diameter.LN and LM are chords. M Z L ZL, ZM, and ZN are radii.

9-8 Circles and Circumference Check It Out: Example 1 Name the circle, a diameter, and three radii. The circle is circle D.DI, DG, and DH are radii. G H D I IG is a diameter. IG and IH are chords.

9-8 Circles and Circumference The distance around a circle is called the circumference. Center Radius Diameter Circumference

9-8 Circles and Circumference The ratio of the circumference to the diameter,, is the same for any circle. This ratio is represented by the Greek letter , which is read “pi.” C d C d = 

9-8 Circles and Circumference The formula for the circumference of a circle is C = d, or C = 2r. The decimal representation of pi starts with and goes on forever without repeating. Most people estimate  using either 3.14 or. 22 7

9-8 Circles and Circumference Additional Example 2: Application A skydiver is laying out a circular target for his next jump. Estimate the circumference of the target by rounding  to 3. C = dC  3 8C  24 ft Write the formula. Replace  with 3 and d with 8. 8 ft

9-8 Circles and Circumference Check It Out: Example 2 A second skydiver is laying out a circular target for his next jump. Estimate the circumference of the target by rounding  to 3. C = dC  3 14C  42 yd Write the formula. Replace  with 3 and d with yd

9-8 Circles and Circumference Additional Example 3A: Using the Formula for the Circumference of a Circle Find the missing value to the nearest hundredth. Use 3.14 for pi. d = 11 ft; C = ? C = dC  C  ft Write the formula. Replace  with 3.14 and d with ft

9-8 Circles and Circumference Additional Example 3B: Using the Formula for the Circumference of a Circle Find each missing value to the nearest hundredth. Use 3.14 for pi. r = 5 cm; C = ? C = 2rC  C  31.4 cm Write the formula. Replace  with 3.14 and r with 5. 5 cm

9-8 Circles and Circumference Additional Example 3C: Using the Formula for the Circumference of a Circle Find each missing value to the nearest hundredth. Use 3.14 for pi. C = cm; d = ? C = d  3.14d7.00 cm  d Write the formula. Replace C with and  with d _______  Divide both sides by 3.14.

9-8 Circles and Circumference Check It Out: Example 3A Find the missing value to the nearest hundredth. Use 3.14 for pi. d = 9 ft; C = ? C = dC  C  ft Write the formula. Replace  with 3.14 and d with 9. 9 ft

9-8 Circles and Circumference Check It Out: Example 3B Find each missing value to the nearest hundredth. Use 3.14 for pi. r = 6 cm; C = ? C = 2rC  C  cm Write the formula. Replace  with 3.14 and r with 6. 6 cm

9-8 Circles and Circumference Check It Out: Example 3C Find each missing value to the nearest hundredth. Use 3.14 for pi. C = cm; d = ? C = d  3.14d6.00 cm  d Write the formula. Replace C with and  with d _______  Divide both sides by 3.14.

9-8 Circles and Circumference Standard Lesson Quiz Lesson Quizzes Lesson Quiz for Student Response Systems

9-8 Circles and Circumference Lesson Quiz Find the circumference of each circle. Use 3.14 for  Find the circumference of a circle with a diameter of 20 feet. Use 3.14 for . C = in.C = in. 8 in ft 3 in.

9-8 Circles and Circumference 1. Identify the circumference of the given circle. Use 3.14 for . A in. B in. C in. D in. Lesson Quiz for Student Response Systems 10 in.

9-8 Circles and Circumference 2. Identify the circumference of the given circle. Use 3.14 for . A in. B in. C in. D in. Lesson Quiz for Student Response Systems 6 in.

9-8 Circles and Circumference 3. Identify the circumference of a circle with a diameter of 26 feet. Use 3.14 for . A ft B ft C ft D ft Lesson Quiz for Student Response Systems