Around the Circle
Part of the Circles section of Coddy's Geometry journey. Lesson 6 of 62.
The circumference of a circle is the distance all the way round it. Measure any circle, big or small, and divide its circumference by its diameter: you always get the same number, a little more than 3. That number is called π (pi), and it is about 3.14159.
So the circumference is π times the diameter, and since the diameter is two radii, C = 2×π×r. Here the radius is 3, so the circumference is 2 × π × 3 = 6π. Leave it as 6π: that is the exact answer. A decimal like 18.85 is only a rounding of it.
To find a circumference on the board, tap inside the circle, choose its circumference, and pick the rule Circumference.
Challenge
Circle O has radius 4. Find its circumference C, exact in terms of π, then press Check.
Try it yourself
Find the marked part of the figure
Test yourselfThis lesson includes a short quiz. Start the lesson to answer it and track your progress.
All lessons in Circles
1Parts of a Circle
Center and RadiusChords and DiametersAll Radii Are EqualDiameter Is Two RadiiRecap: Parts of a Circle4Arcs and Sectors
Arc LengthArea of a SectorFind the Central AngleThe Missing Central AngleRecap: Arcs and Sectors2Circumference
Around the CircleUsing the DiameterRadius From CircumferenceCircumference With LettersRecap: Circumference5Radii Make Isosceles
Two Radii, One TriangleBase Angles From the CenterThe Angle at the CenterIsosceles With LettersRecap: Radii and Triangles8Angles on the Same Arc
Same Arc, Same AngleUsing the Same ArcSpot the Bow TieSame Arc With LettersRecap: Same Arc3Area of a Circle
The Area FormulaArea From the DiameterRadius From the AreaCircumference to AreaRecap: Area of a Circle6Inscribed Angles
Meet the Inscribed AngleHalf the Central AngleTwice the Inscribed AngleThe Big ArcInscribed With LettersRecap: Inscribed Angles