Radius From Circumference
Part of the Circles section of Coddy's Geometry journey. Lesson 8 of 62.
The rule C = 2×π×r works backwards too. If you know the circumference, you can find the radius: divide by 2π. Keep π as a letter and it cancels cleanly.
Here the circumference is 10π. Then 10π = 2 × π × r. Divide both sides by π to get 10 = 2r, so r = 5. The diameter would be 10, the number in front of π. That shortcut works whenever the circumference is written as a number times π.
Challenge
The circumference of circle O is 16π. Find the radius OA, 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