trisection of angle


Given an angle of measure (http://planetmath.org/AngleMeasure) α such that 0<απ2, one can construct an angle of measure α3 using a compass and a ruler (http://planetmath.org/MarkedRuler) with one mark on it as follows:

  1. 1.

    Construct a circle c with the vertex (http://planetmath.org/Vertex5) O of the angle as its center. Label the intersectionsMathworldPlanetmathPlanetmath of this circle with the rays of the angle as A and B. Mark the length OB on the ruler.

    ..OABc
  2. 2.

    Draw the ray AO.

    ..OABc
  3. 3.

    Use the marked ruler to determine Cc and DAO such that CD=OB and B, C, and D are collinearMathworldPlanetmath. Draw the line segmentMathworldPlanetmath BD¯. Then the angle measure of CDO is α3. (The line segment OC¯ is drawn in red. Having this line segment drawn is useful for reference purposes for the justification of the construction.)

    ..OABcCD

Let m denote the measure of an angle. Then this construction is justified by the following:

  • Since AOB is an exterior angleMathworldPlanetmath of BOD, we have that m(AOB)=m(OBD)+m(ODB);

  • Since OC=OB=CD, we have that BOC and OCD are isosceles trianglesMathworldPlanetmath;

  • Since the angles of an isosceles triangle are congruentPlanetmathPlanetmath, m(OBC)=m(OCB) and m(COD)=m(CDO);

  • Since OCB is an exterior angle of OCD, we have that m(OCB)=m(COD)+m(CDO);

  • Note that OBC=OBD and ODB=CDO;

  • Thus,

    α=m(AOB)=m(OBD)+m(ODB)=m(OBC)+m(CDO)=m(OCB)+m(CDO)=m(COD)+m(CDO)+m(CDO)=3m(CDO).

Note that, since angles of measure π6, π3, and π2 are constructible using compass and straightedge, this procedure can be extended to trisect any angle of measure β such that 0<β2π:

  • If 0<βπ2, then use the construction given above.

  • If π2<βπ, then trisect an angle of measure β-π2 and add on an angle of measure π6 to the result.

  • If π<β3π2, then trisect an angle of measure β-π and add on an angle of measure π3 to the result.

  • If 3π2<β2π, then trisect an angle of measure β-3π2 and add on an angle of measure π2 to the result.

This construction is attributed to Archimedes.

References

  • 1 Rotman, Joseph J. A First Course in Abstract Algebra. Upper Saddle River, NJ: Prentice-Hall, 1996.
Title trisection of angle
Canonical name TrisectionOfAngle
Date of creation 2013-03-22 17:16:35
Last modified on 2013-03-22 17:16:35
Owner Wkbj79 (1863)
Last modified by Wkbj79 (1863)
Numerical id 11
Author Wkbj79 (1863)
Entry type Algorithm
Classification msc 01A20
Classification msc 51M15
Related topic VariantsOnCompassAndStraightedgeConstructions