Euclid I.6 (Euclid, Elements, Book I, Proposition 6)

Text: https://www.perseus.tufts.edu/hopper/text?doc=urn%3Acts%3AgreekLit%3Atlg1799.tlg001.perseus-eng1%3A1.prop.6

If in a triangle two angles be equal to one another, the sides which subtend the equal angles will also be equal to one another.

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