Detailed definition
Understanding Obtuse Angle
Obtuse Angle measures more than ninety degrees and less than one hundred eighty degrees. An obtuse angle measures more than 90 degrees and less than 180 degrees. That means it is wider than a right angle but not yet a straight angle.
Obtuse angles are common in triangles, quadrilaterals, and line intersections, so they need to be read as a range rather than as one particular picture. A slanted or inverted obtuse angle is still obtuse if the measure stays in the correct interval.
This concept becomes especially important when comparing angle types. Students who can place obtuse angles accurately between right and straight angles are much less likely to mislabel diagrams in later proof or polygon work.
Key facts
Important ideas to remember
- An obtuse angle measures more than 90 degrees and less than 180 degrees.
- An obtuse angle is greater than ninety degrees and less than one hundred eighty degrees.
- Obtuse describes the interior opening being measured, not the length of either ray.
- A diagram can be rotated or flipped without changing the angle from obtuse to something else.
Where it is used
Where obtuse angle shows up
- Use obtuse-angle classification in polygon and triangle problems where one angle is larger than a right angle.
- Use it when comparing unknown angle values against standard benchmarks such as ninety degrees and one hundred eighty degrees.
- Use it in diagram reading, where a correct classification can determine which theorem or property comes next.
Common mistakes
What to watch out for
- Do not call any wide-looking angle obtuse unless the measure is confirmed to be below one hundred eighty degrees.
- Do not confuse an obtuse angle with a reflex angle, which is measured outside the smaller opening and exceeds one hundred eighty degrees.
- Do not judge the type from ray length, screen size, or perspective distortion.