Diameter Is Two Radii
Part of the Circles section of Coddy's Geometry journey. Lesson 4 of 62.
A diameter runs from the circle, through the center, to the circle again. The center splits it into two radii, so a diameter is exactly twice the radius.
Here the diameter AB is 10 and the radius OC is 5: d = 2r. It works both ways. A radius of 4 gives a diameter of 8, and a diameter of 10 gives a radius of 5. The rule is called Diameter is two radii, and it links the diameter to any radius of the same circle, not only the two halves it is made of.
Challenge
AB is a diameter of circle O and OC is a radius. Find the radius marked ?, 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