Problem:
Given three points , , known to lie on a circle, prove that one can reconstruct the original circle with a straightedge and compass.
Problem:
Given three points , , known to lie on a circle, prove that one can reconstruct the original circle with a straightedge and compass.
Solution:
Here is a suitable procedure: First draw circles centered at and with radius and join their two points of intersection to create the perpendicular bisector of . This line contains all points that have the same distance from and , so the center of a circle through and lies on it. Then, similarly construct the perpendicular bisector of . Because , , and are known to lie on a circle, the two perpendicular bisectors are not parallel (or else no point could have the same distance from , , and ), so they must meet at a point . Draw a circle centered at with radius .
Since lies on both perpendicular bisectors, this circle passes through and . Finally, any other circle through the same three points would have to have its center on both perpendicular bisectors, and hence at . Its radius must equal , implying that it coincides with the constructed circle.