Some expanding mechanisms, made with rack and pinion mechanisms.
Files to make an expanding 3-4-5 triangle: https://www.printables.com/model/1305045-expanding-3-4-5-triangle
You can also use copies of some of the parts to make expanding sticks and square grids.
For more information on the extensors mechanism I mention at the start of the video, see mathmechs.com.
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