In any triangle, there is a unique circle that goes through the three vertices. The centre is called the circumcentre.
Find the midpoints D, E and F of the sides.
Construct the perpendicular bisectors of the sides (the line that goes through the midpoint and is at 90 to the side).
Call the point where the bisector from D and from E meet, point O.
In triangles OBD and OCD, one side is common (OD), BD = DC, and ODB = ODC = 90. So the triangles are congruent (SAS).
So OB = OC.
Similarly OC = OA.
So in triangles OAF and OBF, OA = OB, OF is common and AF = BF, so the triangles are congruent (SSS). So angle OAF = OBF = 90.
So the third bisector goes through O as well.
A circle, centre O, with radius OA will also go through B and C.
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