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Concept
Morley's trisector theorem
Summary
In plane geometry, Morley's trisector theorem states that in any triangle, the three points of intersection of the adjacent angle trisectors form an equilateral triangle, called the first Morley triangle or simply the Morley triangle. The theorem was discovered in 1899 by Anglo-American mathematician Frank Morley. It has various generalizations; in particular, if all the trisectors are intersected, one obtains four other equilateral triangles.
There are many proofs of Morley's theorem, some of which are very technical.
Several early proofs were based on delicate trigonometric calculations. Recent proofs include an algebraic proof by extending the theorem to general fields other than characteristic three, and John Conway's elementary geometry proof. The latter starts with an equilateral triangle and shows that a triangle may be built around it which will be similar to any selected triangle. Morley's theorem does not hold in spherical and hyperbolic geometry.
One proof uses the trigonometric identity
which, by using of the sum of two angles identity, can be shown to be equal to
The last equation can be verified by applying the sum of two angles identity to the left side twice and eliminating the cosine.
Points are constructed on as shown. We have , the sum of any triangle's angles, so Therefore, the angles of triangle are and
From the figure
and
Also from the figure
and
The law of sines applied to triangles and yields
and
Express the height of triangle in two ways
and
where equation (1) was used to replace and in these two equations. Substituting equations (2) and (5) in the equation and equations (3) and (6) in the equation gives
and
Since the numerators are equal
or
Since angle and angle are equal and the sides forming these angles are in the same ratio, triangles and are similar.
Similar angles and equal , and similar angles and equal Similar arguments yield the base angles of triangles and
In particular angle is found to be and from the figure we see that
Substituting yields
where equation (4) was used for angle and therefore
Similarly the other angles of triangle are found to be
The first Morley triangle has side lengths
where R is the circumradius of the original triangle and A, B, and C are the angles of the original triangle.
Official source
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