Definition: 1.12: Acute Angle
And an acute angle (is) one less than a right angle.
Modern Definition
Euclid's definition corresponds to today's formulation.
Example
The angle \(\angle{PQR}\) (marked darkgreen) is acute.
Mentioned in:
Definitions: 1 2 3 4
Proofs: 5 6 7 8
Propositions: 9 10 11 12
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References
Adapted from (subject to copyright, with kind permission)
 Fitzpatrick, Richard: Euclid's "Elements of Geometry"