In Euclid's The Elements, Book 1, Proposition 4, he makes the assumption that one can create an angle between two lines and then construct the same angle from two different lines. I do not see anywhere in the list of definitions, common notions, or postulates that allows for this assumption. When teaching my students this, I do teach them congruent angle construction (with straight edge and compass), but do not understand why Euclid is allowed to make this logical leap. Thanks for any help you can give.
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3This question doesn't belong on Mathoverflow, but might get a good reception on matheducators.stackexchange.com – David E Speyer Jan 29 '20 at 01:46
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1Posted on MESE here. – Joseph O'Rourke Jan 29 '20 at 14:01