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What Is an Obtuse Angle?
Updated Math & Study Help
The usual comparison set
| Type | Measure |
|---|---|
| Acute | greater than 0° and less than 90° |
| Right | exactly 90° |
| Obtuse | greater than 90° and less than 180° |
| Straight | exactly 180° |
| Reflex | greater than 180° and less than 360° |
Examples: 91°, 120°, 150°, and 179° are all obtuse. 90° is not obtuse (it's right). 180° is not obtuse (it's straight).
Obtuse triangle: a triangle with one obtuse angle (and two acute ones). A triangle can't have two obtuse angles, because angles in a triangle already sum to 180° — two angles over 90° would overshoot.
How to spot one in a diagram: if the opening is clearly past the square corner of a right angle, but the two rays aren't in a single straight line, it's obtuse. A protractor reading between 90 and 180 confirms it.
Memory aid: "obtuse" in everyday English means blunt or not sharp — an obtuse angle is the "blunt" one compared with a sharp acute angle.
Nearby questions in this topic include What Is Displacement? and What Is Horizontal?.
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