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AB
Try it on the Geometry BoardUses:Midpoint and perpendicular bisector

Regular polygon on a segment: 12 sides

Builds a regular polygon on a segment, with its corners and sides. Ruler and compass only.

All parts
Part 1 of 8: Equilateral triangle on ABStep 1 of 6Compass

Its top corner X is on the perpendicular bisector. Put the point of your compass on A.

Keys: left and right arrows step, Space plays or pauses, Escape goes back to all constructions.