Euclid’s Proposition I.4
Proposition I.4 presents Euclid’s proof of what is called the “Side-Angle-Side Theorem” in modern schools. The question students often ask is in Classical Geometry, when studying proposition 1.4 is, “How can we just assume the two triangles with two equal sides and a common angle between them?” That does seem to come out of nowhere. These two imagined triangles can be produced using the Elements and Propositions I.1-3. See image above. To summarize: Those who have studied propositions I.1-3 should be able to see how this was done. Follow the letters in alphabetical order. I haven’t formally proven this yet, … Continue