Quadratrix

Uses the curve [Quadratrix] in the file [EUC_Scripts\EUC_Curves\Quadratrix].
The characteristic property of the curve is that the ratio of the arcs ED/BD is equal to the ratio of the lengths FH/BA. The curve can be therefore used to divide an angle to any given ratio.

Switch to the pick-move tool (press CTRL+2), catch E and move on the circle.
Look at the ratios that are equal to each other.

http://users.math.uoc.gr/~pamfilos/