Trisecting a line segment:  The example shown results in trisecting the line segment in red.

The steps involved are:

  1. Construct a circle with the radius the length of the segment at each endpoint (A and B) of the line segment.  ( 2 circles total)
  2. Denote the points at the intersection of the two circles (C and F).
  3. Construct a line between the top point of intersection (C) of the circles and the center of the circles (A and B).
  4. Denote the the points of intersection of this line with the circles (D and E).
  5. From these new points, construct line segments to the center of the other circle. (segments DB and EA)
  6. Construct two line segments from the center of each circle to the bottom point of intersection between the two triangles. (Segments AF and BF)
  7. From the point where the line segments in #5 (DB and AE) intersect the line segments (AF and BF) from #6 (at points G and H) , construct lines to the top point of intersection between the two circles (GC and HC).
  8. Each of these two lines, intersects the original line segment 1/3rd the distance from their respective endpoint. (Points I and J)

If Geometers Sketchpad available:  GSP Trisecting a Line


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