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AB
Try it on the Geometry BoardUses:Midpoint and perpendicular bisectorParallel through a point

Circle of Apollonius

Draws the circle of points whose distances to two points are in a ratio you type. Ruler and compass only.

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Part 1 of 8: Divide AB inside in the ratio 2:1 (1 of 3)Step 1 of 9Ruler

The circle crosses AB where it is divided in the ratio 2:1. You need a second line to step along. Lay the ruler through A at an angle to AB.

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