Hearted Youtube comments on Mathologer (@Mathologer) channel.
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I'm actually surprised and pleased with myself for being able to understand the solution from the clip alone, haha.
Of course it is such a simple that anyone should be able to understand it, was is not for he using a binary number representation, and most people being unfamiliar with binary. If, for instance, he decided for no particular reason to use 0 and 9, everyone who isn't familiar with binary would immediately have an easier time understanding it.
You can also describe the solution without using any numbers at all. You can just use the letters D and U, and show that the only two things that can happen to the sequence of letters is two Ds turn into Us, or a D swaps places with a U to its right.
As a result you either end up with two Ds next to each other, or they are separated by some run of Us. In the latter case you can move the rightmost one until its all the way to the right, at which point the only remaining option is to move the leftmost D closer to the rightmost one until you're back to the former case, meaning you must turn both Ds into Us.
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I have a special relationship to Ptomelys Inequality because it's the first theorem where I came up with an original and beautiful proof of it. And it was actually because of you :D Back then, I was re-watching some video about proofs of Pythagoras and one of the proofs involved scaling the original triangle by the factors a, b and c and then rearranging the parts in a clever way. You can do the same thing with Ptolemys Inquality!
Take any quadrilateral with side lengths a, A, b, B, c, C like in your video but make sure that c is "in between" a and b. Take three copies of the quadrilateral and scale it by a, b and c respectively. Notice that two of your copies now have a side with length ab and diagonals ac and bc respectively. Join them on the side ab. Now notice that you can fit the third copy perfectly on the diagonals of the other two, as the third copy has two side lengths ac and bc too and the angle also matches. Put it in place and look carefully. You will have formed a triangle with side lengths aA, bB and cC. Ptolemys Inquality therefore follows. And the equality will hold exactly when the point lands exactly on the cC side. Which will happen exactly when the opposite angles of the original quadrilateral add up to 180 degrees which is exactly the case when it is circular. qed
I stand by my opinion that this proof is absolutely marvelous and the best proof of Ptolemys Inequality ever. And it's perfect for an animation so if you ever want to show it, I absolutely allow you to do so :)
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