by **Guest** » Sat Mar 19, 2005 1:21 am

It depends on what you have to work with. Two ways:

1) Mathematically: If you have (x,y) coordinates for three points on the circle, you fit the equation:

(x-xc)^2 + (y-yc)^2 = r^2 for the center coordinates (xc, yc) and the radius r. (A fair bit of math)

A graphical form: From the "center" point, construct lines to the other two points. Construct perpendicular bisectors of these lines. They cross in the center.

2) Graphically: From a point outside the circle, construct two tangents (ideally at the point they form about a 90 degree angle). At the points of tangency, construct perpendiculars. They cross in the center of the circle, to the accuracy of your graphical construction.