Given a segment of a circle, to describe the complete circle of which it is a segment.
Proof
Let ABC be the given segment with chord AC and arc through B.
Pick B on the arc; join AB, BC. Bisect AB at D and BC
at E (I.10). At D and E erect perpendiculars to AB and
BC respectively (I.11). By III.3 / III.9 these perpendiculars
both pass through the centre, so their intersection F is the
centre. With centre F and radius FA describe the full circle.