Geometry Circles and Circumference Lesson 6 9 Circles

Geometry: Circles and Circumference Lesson 6 -9

Circles • A circle is the set of all points in a plane that are the same distance from a given point, called the center.

Circles The diameter (d) is the distance across a circle through the center of the circle. The circumference (C) is the distance around a circle. The radius (r) is the distance from the center to any point on the circle.

Circles • Pi is a non-terminating and non-repeating number represented by the Greek letter • (pi) • 3. 14 is often used as an approximation for .

Circles • • Formulas for finding Circumference C = d C = 2 r If you are given the diameter in a problem use the formula with d. If you are given a problem with the radius use the formula with r. • Both formulas find the circumference.

Find Circumference • Which formula for C will you use? • C = 2 r • C = 2 (21) • C = 131. 9 inches 21 in

Your Turn Find the Circumference • Which formula for C will you use? • C = d • C = (4. 5) • C = 14. 1 cm 4. 5 cm

Circles - Area • Formula for finding the area of a circle: 2 • A = r • If you are given the diameter instead of the radius; divide the diameter by 2 to get the radius.

Find the Area • D = 14 m r=? • 14 ÷ 2 = 7 m • A = r 14 m 2 • A = (7 2 ) • A = (49) • A = 153. 9 m 2

Your Turn Find the Area • A = r 2 • A = (5 2 ) 5 ft • A = (25) • A = 78. 5 ft 2

Practice / Homework • Practice: Lesson 6 -9 pg 579 # 1 -4 • (Note the directions say to use 3. 14 for pi. Do not use the pi button on the calculator. Type in the value 3. 14) • Homework: Complete Lesson 6 -9 #2 -22 evens