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4

I bought the product pictured below from Amazon, around four years ago, for about $10. That exact product is no longer available, but if you're willing to search Amazon for "whiteboard compass", then you'll see a number of similar products. It's worked satisfactorily for me, although I don't put it through heavy use (frankly, just one day a year ...


5

This looks like it should work with any (regularly-sized) dry erase marker, since just the drawing tip fits through the holes: [Note: I don't want to link directly to the ebay post since those change over time, and I'm not suggesting this seller. It is currently the first item that Google returns when I search "suction cup draw circle whiteboard".]...


2

The inscribed angle theorem. Sometimes known as the Star Trek Lemma. A generalization of Thale's Theorem.   Wikipedia image


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