Definition: 1.11: Obtuse Angle
An obtuse angle is one greater than a right angle.
Modern Definition
Euclid's definition corresponds to today's formulation.
Example
The angle \(\angle{RQP}\) (marked lightgreen) is obtuse.
In the following figure both angles \(\angle{ABC}\) and \(\angle{CBA}\) are obtuse.
Mentioned in:
Axioms: 1
Definitions: 2 3 4
Proofs: 5 6 7
Propositions: 8 9 10
Thank you to the contributors under CC BYSA 4.0!
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 @Calahan
 @Casey
 @Fitzpatrick
References
Adapted from CC BYSA 3.0 Sources:
 Callahan, Daniel: "Euclidâ€™s 'Elements' Redux" 2014
Adapted from (Public Domain)
 Casey, John: "The First Six Books of the Elements of Euclid"
Adapted from (subject to copyright, with kind permission)
 Fitzpatrick, Richard: Euclid's "Elements of Geometry"