Hearted Youtube comments on Mathologer (@Mathologer) channel.

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  35. 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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