Pleased to Meet You
• Five people meet in a room. Each one
shakes hands with the other four.
• How many handshakes are there?
• (Hint: Draw a picture)
Pleased to Meet You
• There are a total of 10 handshakes.
• Draw five circles to represent the five people. Since each
handshake is created by two people, draw lines between
each of the circles to represent the handshakes. Then,
count the lines to find that there are a total of 10
handshakes.
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 = 14.1cm
4.5cm
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
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