lesson 6-9 pages 275-277 geometry: circles and circumference (and area) lesson check 6-8
TRANSCRIPT
What you really need to know!
A circle is the set of all points in a plane that are the same distance from a given point, called the center.
What you really need to know!
The diameter d is the distance across the circle through its center. The radius r is the distance from the center to any point on the circle.
What you really need to know!
For every circle, C ÷ d is always equal to 3.1415926…. This number is represented by the Greek letter (pi).
What you really need to know!
The circumference C of a circle is equal to its diameter d times , or 2 times its radius r times .
C = d or C = 2r
What you really need to know!
The Area A of a circle is equal to times r2.
A = r2 or A = rr
Demonstration
Example 1:
Find the circumference around the hamster’s running wheel. Round to the nearest tenth.
r = 3 inches
C = 2r
C = 2 • 3.14 • 3
C = 18.8 in
Example 1:
Find the area inside the hamster’s running wheel. Round to the nearest tenth. A = r2
A = •r•r
A = 3.14 • 3 • 3
A = 28.3 in2
Example 2:
Find the circumference of a circle with a diameter of 49 centimeters.
d = 49 cm
r = 24.5 cm
C = 2r
C = 2 3.14 24.5
C = 153.86 cm
49 cm
Example 2:
Find the area of a circle with a diameter of 49 centimeters.
d = 49 cm
r = 24.5 cm
A = rr
A = 3.14 • 24.5 • 24.5
A = 1884.785 cm2
49 cm
1. Find the LCD.2. Rewrite fractions using the LCD.3. Add only the numerators, keeping the LCD.4. Reduce to lowest terms.