Center and Radius
Part of the Circles section of Coddy's Geometry journey. Lesson 1 of 62.
A circle is every point that sits the same distance from one fixed point, its center. That distance is the circle's radius. The word also names any segment from the center to a point on the circle, so a circle has as many radii as you like to draw.
A circle is named by its center: this is circle O. Both OA and OB are radii, because each runs from O to a point on the circle. A segment that starts at O but stops inside the circle is not a radius. Its far end has to be on the circle itself.
Challenge
Three segments are drawn in circle O. Tap the one that is a radius, then press Check.
Try it yourself
Find the right part on 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