NCTM LogoThe Math Forum

Ask Dr. Math: FAQ

  "Impossible" Geometric Constructions   

Dr. Math FAQ || Classic Problems || Formulas || Search Dr. Math || Dr. Math Home

Trisecting an angle

Squaring the circle

Doubling the cube

Three geometric construction problems from antiquity puzzled mathematicians for centuries: the trisection of an angle, squaring the circle, and duplicating the cube. Are these constructions impossible?

Whether these problems are possible or impossible depends on the construction "rules" you follow. In the time of Euclid, the rules for constructing these and other geometric figures allowed the use of only an unmarked straightedge and a collapsible compass. No markings for measuring were permitted on the straightedge (ruler), and the compass could not hold a setting, so it had to be thought of as collapsing when it was not in the process of actually drawing a part of a circle.

Following these rules, the first and third problems were proved impossible by Wantzel in 1837, although their impossibility was already known to Gauss around 1800. The second problem was proved to be impossible by Lindemann in 1882.

The impossibility proofs depend on the fact that the only quantities you can obtain by doing straightedge-and-compass constructions are those you can get from the given quantities by using addition, subtraction, multiplication, division, and by taking square roots. These numbers are called Euclidean numbers, and you can think of them as the numbers that can be obtained by repeatedly solving the quadratic equation.

These three problems require either taking a cube root or constructing pi. A cube root is not a Euclidean number, and Lindemann showed that pi is a transcendental number, which means that it is not the root of an algebraic equation with integer coefficients, making it too non-Euclidean.


Trisection of an angle

Given an angle, construct an angle one third as large. The problem must be solved for an arbitrary angle. (Some angles, such as 90 degrees, can be trisected easily.)

Angle trisection using straightedge and compass is equivalent to solving a cubic equation. Even when it is restricted to integer coefficients, the construction can only calculate the solution of a limited set of such equations. Since it can be shown that the equation for trisecting a 60-degree angle cannot be solved using only an unmarked ruler and compass, a general method of trisecting an angle is not possible.

Constructions, Discussions, Proofs

   General Discussions    How to trisect an angle using a compass and MARKED straightedge And see, from the discussion group geometry-puzzles:

  Squaring (Quadrature of) the circle

Given the radius of a circle, construct the side of a square of the same area. (Or, construct a square equal in area to the area of any given circle.)

A circle and square have an equal area only if the ratio between a side of the square and a radius of the circle equals the square root of pi. Lindemann proved that two line segments cannot be constructed to have lengths in this ratio and therefore this method can not square the circle.

Constructions, Discussions, Proofs

   General Discussions    Proofs that the circle cannot be squared

  Doubling (or Duplicating) the cube

Given the side of a cube, construct the side of a cube that has twice the volume. (Also called the Delian problem.)

Doubling a cube whose edge equals 1 yields the equation x^3 = 2, whose solution (the length of a side of the larger cube) is the cube root of 2. The problem cannot be solved because the so-called Delian Constant (the cube root of 2) is not a Euclidean number.

Constructions, Discussions, Proofs

   General Discussions    How to double a cube with a torus and a cone    Biographies

Submit your own question to Dr. Math

[Privacy Policy] [Terms of Use]

Math Forum Home || Math Library || Quick Reference || Math Forum Search

Ask Dr. Math ®
© 1994-2017 The Math Forum at NCTM. All rights reserved.