Diameter - Geometry Made Easy - What is the diameter of a circle? How do we know the diameter if we only have the radius of a circle?
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Diameter of a Circle

Radius

The diameter of a circle is the distance across the circle at the widest part of the circle. The diameter is always equal to twice the radius of the circle, because the radius starts from the center of the circle and goes straight to the edge.

Learn by doing: measure the diameter of some circles in your house
Radius
Area of a Circle
More about Geometry

Bibliography and further reading about circles:

More Geometry
More about Math
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Professor Carr

Karen Eva Carr, PhD.
Assoc. Professor Emerita, History
Portland State University

Professor Carr holds a B.A. with high honors from Cornell University in classics and archaeology, and her M.A. and PhD. from the University of Michigan in Classical Art and Archaeology. She has excavated in Scotland, Cyprus, Greece, Israel, and Tunisia, and she has been teaching history to university students for a very long time.

Professor Carr's PSU page

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Happy New Year! Welcome back! Get ready for Martin Luther King day with these articles about medieval Africa, slavery, the Civil War, emancipation, the civil rights movement, and Martin Luther King Jr. himself. More about King here...