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Circumference & Pi
3.14
Find the perimeter of the trapezoid.
Find the area of the trapezoid
A circle has a diameter of 30 feet. What is the radius?
The radius of a circle is 7 inches. What is the diameter?
96
9
17.5
5
30 feet
7 in
Circumference: The distance around the outside of a circle or think the perimeter of a circle
Pi (π):
The ratio of the circumference to diameter.
C = πd = 2πr
π(d) = π(2r)
= 2πr
Circumference of a Circle
π 3.14
Videos
SAV Lesson 4 ~ Circumference:
Simple (pi = 3) : http://www.onlinemathlearning.com/circle-circumference.html
Geeky: http://www.youtube.com/watch?v=ORN7qLkuKOk&NR=1
Nerd exact pi: http://www.youtube.com/watch?v=cuhSPfyuulM&NR=1
Find the circumference of the circle.Use 3.14 for π.
Write the circumference formula that uses diameter. C = πd
Substitute the known values. C ≈ (3.14)(8) Multiply. C ≈ 25.12
The circumference of H is about 25.12 ⊙ mm.
Find the circumference of the circle.Use 3.14 for π.
Write the circumference formula using the radius.C = 2πr
Substitute the known values. C ≈ 2(3.14)(2.5)
Multiply. C ≈ 15.7
The circumference of J is about 15.7 feet.⊙
Natalya walked with her friend around a circular track. The circumference of the track was 188.4 meters. Find the approximate length of the track’s radius. Use 3.14 for π.
Write the circumference formula. C = 2πr Substitute the known values. 188.4 ≈ 2(3.14)r Multiply. 188.4 ≈ 6.28r Divide by 6.28 on each side.
30 ≈ r
The radius of the track was approximately 30 m.
28.6
28.6
28.6
4.188 r
Use 3.14 for in each problem.
1. Find the circumference of a circle with a diameter of 5 inches.
2. Find the circumference of a circle with a radius of 5 inches.
3. The circumference of a circle is 39.25 mm. What is the diameter?
Why do you think the word “circumference” is used instead of “perimeter” for the distance around a circle?
What is pi ()?