In his 5 postulates Euclid said:"All the right angles are equal". What he really meant by that ? Does he meant that we can impose any right angle on each other or the meaning there was that if you find this angle through the length of arc and radius of the circle then you get the same number ? I also heard that this postulate can be proven (and I remembered that I saw such a question there, but now I can't find it). So what is the proof (based on the definition of a right angle like right angle is an angle which is equal to its adjacent angle)
2026-03-24 22:08:17.1774390097
What means "equal" in Euclid 4th postulate?
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As mentioned by Moische Cohen equal should be interpreted as congruent. Further congruent is an undefined term in Hilbert's geometry. Note the there are two different undefined congruence notions, one for segments the other for angles.