
Angles
Euclid’s fourth postulate is that all right angles are congruent. An acute angle is less than a right angle, while an obtuse angle is more. The angles 30°, 45°, 60° and 120° get lots of usage and are constructible. Curiously, the 20° angle is not constructible.
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Euclid Book 1 Proposition 9
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Euclid Book 1 Proposition 13
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Euclid Book 1 Proposition 14
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Euclid Book 1 Proposition 15
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Euclid Book 1 Proposition 16
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Euclid Book 1 Proposition 17
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Euclid Book 1 Proposition 23
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Euclid Book 1 Proposition 27
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Euclid Book 1 Proposition 28a
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Euclid Book 1 Proposition 28b
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Euclid Book 1 Proposition 29
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Euclid Book 1 Proposition 32
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Euclid Book 3 Proposition 20
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Euclid Book 3 Proposition 21
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Euclid Book 3 Proposition 22
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Euclid Book 3 Proposition 31a
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Euclid Book 3 Proposition 31b
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Euclid Book 3 Proposition 31c
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Euclid Book 3 Proposition 32
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Euclid Book 3 Proposition 33
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Euclid Book 3 Proposition 34