'No slack, Alice!' says Grace Grace mimicked Mr Dodgson: ‘'Take two different positive odd numbers with no shared prime factor. Multiply the larger by their sum, giving the perimeter of a distinct right-angled triangle with whole-number sides. Only such values and their multiples work.'' Alice created a loop from part of a 1m thread. She was able to pull it tightly on a pinboard into a right-angled triangle with whole-number cm sides in two ways (with different-shaped triangles). Alice then pulled the same loop tight over the thin polar spike of the classroom globe, down two meridians and along the equator.

https://www.thetimes.co.uk/article/teaser-3181-cnhf0gqdb

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