Morley's Theorem, a Proof

Nolan L Aljaddou
April 11, 2009

I believe I have discovered the simplest and most fundamental proof of Morley's trisector theorem. If you find it satisfactory, you may post it if you wish. I would be interested in your feedback either way.

The Fundamental Proof of Morley's trisector theorem:

All triangle endpoints are formed by the intersection of three distinct lines. That is, for three triangle sides A, B, and C, the endpoints are generated as follows: ∪{(A∩B), (B∩C), (C∩A)}. The endpoints are generated not only by the intersection of each pair of lines, but equivalently by the intersection of the lengths of each of their reversed angular projections, thus each may be equally replaced by their equivalent angles in the set, which is stated as ∩{∪[α, β, γ]}.

The trisection of a triangle's angles divides them each into three, and since their original total sum is 180°, the sum of one of each of their trisections - being a third of each original angle - is collectively equal to one third of the total of 180°, which is 60°. These three distinct angles whose sum is 60° may be denoted as a, b, and c.

Since each angle of an equilateral triangle is 60°, each of its angles can be divided into three parts which are equal to a, b, and c. Therefore, the equilateral triangle itself is generated by the intersection of the union of these three angles, three times. That is: ∩{[∪(a, b, c), ∪(a, b, c), ∪(a, b, c)]} generates the endpoints of the equilateral triangle. The union of the three angles is geometrically equivalent to the addition of the three angles; that is, ∪(a, b, c) = (a + b + c), therefore the previous intersection is equivalent to: ∩{∪[(a + b + c), (a + b + c), (a + b + c)]}, which itself is geometrically equivalent to ∩{(a + b + c) + (a + b + c) + (a + b + c)}, which reduces to ∩{3(a + b + c)} or ∩{(3a + 3b + 3c)} = ∩{∪[3a, 3b, 3c]}, which is the generator of the endpoints of the original triangle. That is, the intersection of these three original angles not only generates the original triangle, but the intersection of their trisection also generates the endpoints of an equilateral triangle - which mutually generate the distinct, reversely projected angular trisections adjacent to one another.

QED


Now a commentary by AlexB from CTK.

Mr. Aljaddou's message brought to mind a construction of a family of equilateral triangles as a consequence of the Lighthouse theorem. The proof of the Lighthouse theorem is fairly elementary, based primarily on a theorem about inscribed angles. The simplicity, not similarity, of the two arguments suggested a possible link between the two. However, I needed clarification and a short correspondence ensued which I report below for the benefit of the reader. To me, the hope of a simple proof proved to be a mirage of which I had informed Mr. Aljaddou. Mr. Aljaddou nonetheless expressed the desire to have his proof made public. (The latest version of it appears on a separate page.) Comments are welcome and should be posted at the bottom of that page.

AlexB

  I must admit I did not understand a word, starting with the " reversed angular projections". Out of curiosity I did make an effort to get to the bottom of it, but failed miserably. Should all the unions and intersections be understood geometrically as unions and intersections of plane shapes?

N L A

  Yes, it was in fact a difficult concept to put into words as well. A typical angular projection begins with a point, with two linear lengths extending outwards; by reversed angular projection, I meant two linear lengths converging to a point, so in effect the reverse of a traditionally viewed projection. The essence is to replace a typical intersection of lines with the angle they produce, and reversely generate them with its projection; in this complementary representation the information about the relation between the lines, angles, and points of the triangles can be properly interchanged and translated. Also yes, in this particular case, all the set unions and intersections directly correspond to geometric unions and intersections. Please send me any suggestions you may have with regard to re-wording some concepts in a potentially more universally comprehensible manner, although I think a proper diagrammatic representation could also potentially achieve that.

AlexB

 

Well, the diagram may be quite useful.

Let's check the first paragraph:

"All triangle endpoints are formed by the intersection of three distinct lines. That is, for three triangle sides A, B, and C, the endpoints are generated as follows: ∪{(A∩B) , (B∩C) , (C∩A)}. The endpoints are generated not only by the intersection of each pair of lines, but equivalently by the intersection of the lengths of each of their reversed angular projections, thus each may be equally replaced by their equivalent angles in the set, which is stated as ∪{∩[α, β, γ]}."

  1. I believe understand ∪{(A∩B) , (B∩C) , (C∩A)}, viz., three lines intersect in three points and a triangle is thought of as the union of three points. This is a strange concept that omits the sides of the triangle, but I can provisionally accept it.

  2. I believe I understand the concept of the "reverse perspective." I do not understand how does it swap the roles of angles and lines. A diagram may help here.

  3. I do not understand the distinction between lines and lengths. If there is any it should be explained. If there is none, the terminology should be more reserved.

  4. I do not understand ∪{∩[α, β, γ]}.

    α, β, γ are the angles of a triangle understood geometrically. ∪{∩[α, β, γ]} is the union of three plane regions. If that is right whose intersection is taken? What is left to intersect?

N L A

 

With regard to the first case, the points are considered exclusively, as a given triangle's sides may be subtracted while retaining its endpoints without losing its shape, as line segments connecting the points are secondary information which depend only the existence of the two mutual endpoints which connect them. This lack of dependence on limited line segments defining the triangle allows more freedom to illustrate only the key connections.

Second, rather than considering an angle as a secondary product dependent on the intersection of two lines, the reverse perspective of having a given angle first, and then resultantly generating the two lines it projects changes a given angle from a dependent variable into an independent variable, which is key to showing how a given set of angular relations can by itself produce another given set of line intersections.

Third, a length refers to the lines of intersection only in the sense in which they are the dependent variable generated by a given angle's projection, and they are different in this sense from the line segments which bound the triangle in that they are considered to extend infinitely and are only limited by their mutual intersection with each other, as in the case of a triangle. For example, a triangle with vertex A has angle a, and vertex B has angle b, and if it is viewed as being relatively upright with A at the top and B to the bottom right, the left length of B intersects with the left length of A, which produces the third vertex C; B's left length would have gone on infinitely without the intersection with the left length of A; a triangle is defined by the intersection of its angle projections in this way, and the lengths are meant to extend indefinitely until intersection with another length for the later purposes of the proof, with the term length reserved to specifically refer only to the line of the angular projection for clarity purposes.

Fourth, as mentioned above, it is the intersection of the angular projections which defines the endpoints of the triangle. A's angle "a" projection extends on the right side to coincide with vertex B's angle "b" projection on the right side, and vertex C's angle "c" projects to the right side to coincide with the projection of "a" and the origin point of the projection of angle "b" on the right side; it is at that point that all three angle projections' lengths coincide and intersect, and the same is for the other two vertices of the triangle, and the intersection of all three projections then occurs only at those three points, which are the three endpoints of the triangle.

The rest of the proof follows incontestably from the proper establishment of those principles, which themselves simply amount to a new, more elastic perspective of equally defining otherwise traditional concepts. The main insight was the realization that the sum of one of each of the trisected angles of any given triangle always add up to 60 - (3a + 3b + 3c)/3 = a + b + c = 180/3 = 60 - 60 being the measure of each angle in an equilateral triangle as well; thus these trisected angles together can represent each of the equilateral triangle's angles, which becomes 3(a + b + c) = 3a + 3b + 3c, which are the original angles. The key was simply to show the proper transformations which brought this about, part of which involved transforming the angle of a triangle from a dependent variable to an independent variable. Please let me know if you think the information presented here makes the proof more clear.

AlexB

 

...

Second, I can live with a notion of angle as a combination of two intersecting lines or rays, if you prefer. Is that any different from your idea of reversed angular projection? Somehow, the word "projection" has in my mind an association that requires to indicate what is projected and to where. There is no gain in using that word. So you like to see a triangle as an intersection of angles, do you? Or just the lines forming those angles? What does ∪{∩[α, β, γ]} relate to? This union/intersection symbolism appears to play an important role in your way of thinking. Just explain please this a step at a time:

What is ∩[α, β, γ]?
What is ∪{∩[α, β, γ]}, as related to the above?

Third, if you do not mind, I believe one can talk without confusion of "sides of a triangle" and "side lines of a triangle". A side is a line segment; a side line is a line (an object extending indefinitely in two directions). You use the word "length" in the sense of "expanse" which you could but which is not strictly necessary. "Length" has commonly a different association. It's rather a number or a measure. This is quite unlike the word "circumference" that is used both to denote the length of a circle's border and the border itself.

Once again, thinking of an angle as projecting its side lines is unnatural to my world view. Why? Let it just have, possess, consist of, comprise, or whatever else, its side lines. Angle may well be an abstract, Platonic object with incarnations in a common plane - other geometric objects constitute distinct abstract concepts and have their plebeian realizations that we approximate by drawing in the plane. It is unusual (and, to me, confusing) to refer to such an association of abstract and concrete a projection.

N L A

  Two lines intersect to produce an angle; this is identical to saying that a unique angle geometrically corresponds to the intersection of two lines. Two lines intersecting and an angle are then an equivalence relation. Therefore, just as easily as two lines intersecting produces a dependent angle, the reverse is equally true, that a given angle produces a dependent set of lines. It is extremely abstract, and counter-intuitive, but that's precisely what is needed to prove this result in the most fundamental way. The meaning of ∩[α, β, γ] is most clear when visualized. As proven above, an angle generates two lines just as equally as two lines generate an angle; that is what each angle of the set represents: not only their angle, but the two lines they equally generate. When these angles are conjoined to form a triangle, they each share one another's adjacent line, thus this is a geometric "union" of each angle with the other, and forms a collective union set of the angles; the union of all of them then produces the triangle. The "intersection", is the point at which they precisely intersect with each other, all together, and form the vertices; therefore this set is the set of triangle endpoints. It's a new spin on fundamental notation, entirely valid, and by virtue of itself necessary for the most elementary understanding of this topic. And with regard to the terminology of angle projections, each angle is in fact projecting onto something; namely, an infinite number of possible points, the only ones of interest being the points of intersection with the others. The same applies for the use of the term length. Please let me know if this is more clear.

At this point I felt there was no need for further clarifications. The final version of Mr. Aljaddou's proof appears elsewhere.


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