Geometry
Obtuse Angles – Larger Than a Right Angle
An obtuse angle is any angle that measures more than 90° but less than 180° – wider and more “open” than a right angle, but not yet a flat line. The word “obtuse” comes from the Latin obtusus, meaning “blunt” or “dull” – the exact opposite of “acute” (sharp), which makes the two terms a natural, easy-to-remember pair.
Obtuse angles sit directly between right angles and straight angles on the number line of possible angle measures, making them the natural next step after mastering right angles.
Recognising an Obtuse Angle
An obtuse angle measures strictly between 90° and 180°. It looks wider and more “open” than a right angle's square corner.
Is a 135° angle obtuse?
135° is between 90° and 180°, so yes, it is obtuse.
Real-Life Application
- Reclining chairs: a comfortable recline often forms an obtuse angle at the hinge.
- Open laptop screens: a laptop opened wide beyond 90° forms an obtuse angle.
- Roof designs: gently sloped (low-pitch) roofs often meet at an obtuse angle.
Key Takeaways
- An obtuse angle is between 90° and 180°.
- “Obtuse” comes from the Latin word for “blunt,” the opposite of “acute.”
- Obtuse angles sit between right angles and straight angles.
Practice: Obtuse Angles
Obtuse Angles