Using the Diameter
Part of the Circles section of Coddy's Geometry journey. Lesson 7 of 62.
Since π is the circumference divided by the diameter, the circumference is π times the diameter: C = π×d. When a figure gives you the diameter, you can use it straight away, without halving it first.
Here the diameter is 7, so the circumference is π × 7 = 7π. Working from the radius gives the same: the radius is 3.5, and 2 × π × 3.5 = 7π. The two forms are one rule, because the diameter is two radii.
Challenge
BA is a diameter of circle O. Find the 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