compass and straightedge construction of regular triangle


One can construct a regular triangle with sides of a given length s using compass and straightedge as follows:

  1. 1.

    Draw a line segmentMathworldPlanetmath of length s. Label its endpoints P and Q.

    .PQ
  2. 2.

    Draw an arc of the circle with center P and radius PQ¯.

    .PQ
  3. 3.

    Draw an arc of the circle with center Q and radius PQ¯ to find a point R where it intersects the arc from the previous step.

    PQR
  4. 4.

    Draw the regular triangle PQR.

    PQR

This construction is justified by the following:

  • PQ¯PR¯ since they are both radii of the circle from step 2;

  • PQ¯QR¯ since they are both radii of the circle from step 3;

  • Thus, PQR is an equilateral triangleMathworldPlanetmath;

  • In Euclidean geometryMathworldPlanetmath, any equilateral triangle is automatically a regular triangle. Therefore, PQR is a regular triangle.

This construction is based off of the one that Euclid provides in The Elements as the first propositionPlanetmathPlanetmathPlanetmath of the first book. Please see http://planetmath.org/?op=getmsg;id=15600this post for more details.

This construction also yields a method for constructing a 60 angle using compass and straightedge.

Note that, with the exception of actually drawing the sides of the triangleMathworldPlanetmath, only the compass was used in this construction. Since regular triangles tessellate, repeated use of this construction provides a way to find infinitely many points on a line given two points on a line using just a compass.

If you are interested in seeing the rules for compass and straightedge constructions, click on the provided.

Title compass and straightedge construction of regular triangle
Canonical name CompassAndStraightedgeConstructionOfRegularTriangle
Date of creation 2013-03-22 17:19:10
Last modified on 2013-03-22 17:19:10
Owner Wkbj79 (1863)
Last modified by Wkbj79 (1863)
Numerical id 10
Author Wkbj79 (1863)
Entry type AlgorithmMathworldPlanetmath
Classification msc 51M15
Classification msc 51-00
Synonym compass and straightedge construction of equilateral triangle
Synonym compass and straightedge construction of equiangular triangle
Defines compass and straightedge construction of 60 angle