Another great mathematical problem: Quadrisection of a disc

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Discussion Overview

The discussion centers around the mathematical problem of quadrisection of a disc, which involves dissecting a disk into four parts of equal area using three chords originating from a single point on the boundary, one of which is a diameter. The conversation explores the implications of this problem in relation to classical geometric constructions and the limitations imposed by using only a straightedge and compass.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants highlight the connection between quadrisection and other classical problems like squaring a circle and trisection of an angle.
  • One participant presents a mathematical formulation involving the equation $$\dfrac{\pi}{2}=\alpha -\sin(\alpha)$$ and suggests that solving for ##\alpha## yields a value approximately equal to 2.31, which is not a Galois extension of degree ##2^n##.
  • Another participant questions whether using additional tools, such as an angle of the Dottie number, would allow for the division of a disk into an arbitrary number of equal area parts with chords.
  • There is mention of the potential solvability of trisection using an Archimedean spiral if additional tools are permitted, but uncertainty remains regarding the original problem's complexity even with such tools.
  • Participants express that the equation for ##\alpha - \sin(\alpha)## is particularly challenging to solve.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the solvability of the quadrisection problem with the given constraints, and multiple competing views regarding the use of additional tools and their implications are present.

Contextual Notes

The discussion highlights limitations related to the assumptions of classical geometric construction methods and the complexity of the mathematical expressions involved.

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Along with the problem of squaring a circle and trisection of an angle, there is one more great problem: quarisection of a disc.

You have a disk and have to dissect it into four parts of equal area with three chords coming from the same point on the disc's boundary (one of these chords is a diameter).

What makes this problem impossible to solve with straightedge and compass?
 
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"quadrisection", for those of us who have to Google the problem. :wink:
 
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Anixx said:
Along with the problem of squaring a circle and trisection of an angle, there is one more great problem: quarisection of a disc.

You have a disk and have to dissect it into four parts of equal area with three chords coming from the same point on the disc's boundary (one of these chords is a diameter).

What makes this problem impossible to solve with straightedge and compass?
In order to do this, you have to solve
1689717515785.png

$$
A= \dfrac{ \pi r^2}{4}= \dfrac{r^2}{2}(\alpha -\sin(\alpha)) \Longleftrightarrow \dfrac{\pi}{2}=\alpha -\sin(\alpha)
$$
which is something like ##\alpha \approx 2.31## and this number is nowhere even near a Galois extension of degree ##2^n.##
 
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fresh_42 said:
In order to do this, you have to solve
View attachment 329405
$$
A= \dfrac{ \pi r^2}{4}= \dfrac{r^2}{2}(\alpha -\sin(\alpha)) \Longleftrightarrow \dfrac{\pi}{2}=\alpha -\sin(\alpha)
$$
which is something like ##\alpha \approx 2.31## and this number is nowhere even near a Galois extension of degree ##2^n.##
If we have a straightangle, compass and an angle of Dottie number available, can we divide a disk into arbitrary number of parts of equal area with chords?

What if we have only interval of Dottie number and no angle?
 
Things become completely different if additional tools can be used. IIRC then trisection becomes solvable with the help of an Archimedean spiral.

I don't know anything about the problem here with any auxiliary weapons. However, solving the equation for ##\alpha## looks rather difficult, even with additional tools. ##\alpha - \sin(\alpha)## is very inconvenient.
 
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