Hearted Youtube comments on Mathologer (@Mathologer) channel.
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Very nice geometric proofs. It might be also interesting to look at it a bit more algebraically. Let's say a',b',c' are the big parts (as in 2/3) of the respective medians. From Steiner theorem (or law of cosines) we have a'^2=-a^2/6+b^2/3+c^2/3 and similar equalities hold for b', c'. So if we represent the triangle by the vector (a^2,b^2,c^2), "folding" is just a matrix multiplication
(a'^2,b'^2,c'^2)=M*(a^2,b^2,c^2),
where
M=[[-1/6,1/3,1/3],[1/3,-1/6,1/3],[1/3,1/3,-1/6]].
Since M^2=I/4 (where I is the identity matrix), folding twice means making the squares of sides 4x smaller, i.e. scaling the triangle down by a factor of 2.
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