Three Points Determine a Circle
Three points determine a circle. This shows how to construct the center of the circle. Move the points A, B, C and make different circles.
http://en.wikipedia.org/wiki/Circumscribed_circle
http://en.wikipedia.org/wiki/File:Circumcentre.svg
http://en.wikipedia.org/wiki/File:Triangle_circumcenter_alternate_construction.png
Again,
http://en.wikipedia.org/wiki/Circumscribed_circle
http://en.wikipedia.org/wiki/File:Circumcentre.svg
http://en.wikipedia.org/wiki/File:Triangle_circumcenter_alternate_construction.png
These links are useful.