Definition:Obtuse Angle
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Definition
An obtuse angle is an angle which has a measurement between those of a right angle and a straight angle.
In the words of Euclid:
- An obtuse angle is an angle greater than a right angle.
(The Elements: Book $\text{I}$: Definition $11$)
Note that Euclid did not consider the concept of reflex angles.
Measurement of Obtuse Angle
An obtuse angle measures $\theta$, where:
- $90 \degrees < \theta < 180 \degrees$
or:
- $\dfrac \pi 2 < \theta < \pi$
Sources
- 1989: Ephraim J. Borowski and Jonathan M. Borwein: Dictionary of Mathematics ... (previous) ... (next): obtuse: 1.
- 1998: David Nelson: The Penguin Dictionary of Mathematics (2nd ed.) ... (previous) ... (next): angle
- 1998: David Nelson: The Penguin Dictionary of Mathematics (2nd ed.) ... (previous) ... (next): obtuse angle
- 2008: David Nelson: The Penguin Dictionary of Mathematics (4th ed.) ... (previous) ... (next): angle
- 2008: David Nelson: The Penguin Dictionary of Mathematics (4th ed.) ... (previous) ... (next): obtuse angle
- 2014: Christopher Clapham and James Nicholson: The Concise Oxford Dictionary of Mathematics (5th ed.) ... (previous) ... (next): obtuse angle