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PBS
Pbs Learning Media: Finding the Circle's Center
In this video segment [3:10] from Cyberchase, the CyberSquad must find the exact center of a circle in order to save Motherboard.
National Council of Teachers of Mathematics
The Math Forum: Elementary Geometry Circles
The Math Forum offers an archive of question and answer sessions on circles and how they relate to geometry.
Other
Paper Plate Education: Serving the Universe on a Paper Plate
Excellent resource for "paper-plate" activities that help simplify various astronomy topics. Learn how to build a "Moon Finder" and "Planet Pointer," and create a paper-plate model that explains the Transit of Venus.
PBS
Pbs Learning Media: Tricking Hacker
In this video segment from Cyberchase, the CyberSquad must reclaim the Totally Rad Ring by measuring its radius to create a perfect duplicate.