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Bell Work

Bell Work. 26 cm. 4.5 mm. Radius = ____________ Diameter = ___________. 13 cm. Radius = ____________ Diameter = ___________. 4.5 mm. 26 cm. 9 mm. Review of Vocabulary. Circle: The set of points that are the same distance (equidistant) from a given point called the center .

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Bell Work

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  1. Bell Work 26 cm 4.5 mm Radius = ____________ Diameter = ___________ 13 cm Radius = ____________ Diameter = ___________ 4.5 mm 26 cm 9 mm

  2. Review of Vocabulary • Circle: • The set of points that are the same distance (equidistant) from a given point called the center. • Radius: • The distance from the center of a circle to any point on the circle. • Diameter: • The distance across a circle through the center. • REMEMBER: The diameter is TWICE the radius d = 2r

  3. Circumference

  4. Circumference A Centre A circle O The distance around a circle is called its circumference.

  5. Activity Directions • Each of you will receive a ruler, string, and worksheet. • In your groups measure the circumference and diameter of each object. Record your measurements in the chart. • Measure the circumference by wrapping the string around the object. Mark on the string where the two ends meet, and measure the string with your ruler. • In the last column, do the following calculation: • Always round to the nearest hundredth.

  6. What Did You Find Out? • When you take the circumference divided by the diameter you should always get the same number (3.14). • 3.14 is an approximation for π. • π is considered an irrational number, so it never ends. • We can use this ratio to find the formula for the circumference of a circle. diameter • • diameter

  7. Circumference Formulas When the diameter is given, use the following formula: C = π∙d When the radius is given, use the following formula: C = 2∙π∙r

  8. Example Julia wants to find the distance around the circular track at her school. She knows it has a diameter of 0.5 miles. What is the circumference of the track? C = πd C = (3.14)(0.5) C =1.57 miles REMEMBER: We use 3.14 for π. Always round to the nearest hundredth.

  9. Example John wants to build a fence around his circular lawn. He knows the distance from the center to the edge of the lawn is 35 feet. How much fencing does he need to buy? C = 2πr C = (2)(3.14)(35) C =219.8 ft He needs to buy 220 feet of fencing.

  10. What if we wanted to find the radius and diameter of a circle given the circumference? C r = 2π 12 2(3.14) Use π = 3.14 to find the radius of this circle. C = 2πr 12 cm How can we rearrange this to make the radiusthe subject of the formula? r= ? r = 1.91 cm We can also find the diameter since d = 2r Always round your answer to the nearest hundredth. d = 3.82 cm

  11. Example Lily is drawing plans for a circular fountain. The circumference of the fountain is 63 ft. What is its approximate diameter? C = πd 63= 3.14d d = d =20.06 ft REMEMBER: We use 3.14 for π. Always round to the nearest hundredth.

  12. Practice:Circumference Worksheet

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