1. Draw the axis of symmetry.
2. Draw an angle of 45° to both sides of the axis. The legs of these 2 angles are a.
3. Draw parallels at both sides of the axis of symmetry in a distance of .
4. Draw a circle with radius r = a from the endpoints of the legs of the 45°-legs.
5. The points of intersection of these two circles and the parallels are the two missing vertices of the pentagon.