Ruler and Compass Construction 3
Perpendicular to Line Through a Point On the Line
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1. Arbitrary circle at C intersecting line AB at D,E
2. Circle centered at D, radius over DC
3. Circle centered at E, same radius
4. Intersection point F
5. Perpendicular line FC
Why it works:
Point C is equidistant from D and E, hence C is on the perpendicular bisector DE, which is FG.