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# Circles

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### Circles

1. 1. Area of Circlesand Partsof Circles
2. 2. Radius• The radius of a circle is from the center to the outer edge of the circle.• The radius is one half of the diameter as shown here. r
3. 3. Diameter• The diameter goes A from one edge of the circle to the C D other.• The diameters is twice the radius or B r2. m CD = 2.17 cm• AB
4. 4. Pi π• Pi is ≈ 3.14• Pi is represented by the symbol π
5. 5. Parts of a circle A In circle O, OB is a radiusO AC is a diameter BC
6. 6. Parts of a circle A The distance around circle O is called theO circumference of B the circle. It is similar to the perimeter of aC polygon.
7. 7. The number π A The ratio of the circumference of O a circle to its diameter is the number π. m CA = 5.8982 cm CCircumference OB = 18.5299 cm(Circumference OB) = 3.1416 m CA
8. 8. Finding the Circumference Since we know d that C/d = π we can solve for C. r C (d ) = π(d) dTherefore: C = π d or C = 2π r
9. 9. PracticeFinding the Circumference d = 46 cm d Find the r circumference. C = 46π cm or 144.51 cm
10. 10. PracticeFinding the Circumference d = 2.8 m d Find the r circumference. C = 2.8π m or 8.80 m
11. 11. PracticeFinding the Circumference r = 18 cm d Find the r circumference. C = 36π cm or 113.10 cm
12. 12. Finding the area of a circle The area of a circle is found r using the formula A = πr2
13. 13. Practice Finding the area of a circle A A = πr2 C D A = π(2.17)2 A = 4.7089π cm 2 B or 14.79 cm 2m CD = 2.17 cm
14. 14. PracticeFinding the area of a circle A A = πr2 C D A = π(5.19/2)2 A = 6.734π cm2 Bm AB = 5.19 cm or 21.16 cm2