Hearted Youtube comments on Mathologer (@Mathologer) channel.
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I did a bit of trigonometry to express the six angles with the coloured dots in terms of the angles of the given triangle. Here's what I figured out. (I'm sure this is known to the triangle experts.)
With the usual notation, let's call A, B, C the points of the triangle, a, b, c the edges and alf, bet, gam the angles. The median through A divides alf into the angles alf_b and alf_c (to the side of the edges b and c respectively). Similarly, bet=bet_c+bet_a and gam=gam_a+gam_b.
With this, one gets:
cot alf_b = 2 cot alf + cot gam,
cot alf_c = 2 cot alf + cot bet
and two similar pairs of equations. (The proof uses the law of sines and the addition formula for cot.) Btw., it can be checked that cot(alf_b+alf_c)=cot(alf).
Now the folded triangle has angles
alf_F = bet_a + gam_a,
bet_F = gam_b + alf_b,
gam_F = alf_c + bet_c,
and one obtains
cot alf_F=(-cot alf + 2 cot bet + 2 cot gam)/3
and two similar expressions for cot bet_F and cot gam_F, i.e. a linear relation between the cotangents of the angles!
So, if one forms a 3-vector from the cotangents of the angles, then the folding operation from the video is the multiplication of this vector with the 3×3-matrix M which has -1/3 on the diagonal and +2/3 in all other entries. This matrix satisfies M^2=1, reflecting the fact that folding twice reproduces the triangle up to size.
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