What Is an Obtuse Angle?

What makes the opening obtuse

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.

An angle measuring 100 degrees is obtuse, and one measuring 170 degrees is obtuse too. They may look very different, but both belong to the same range. Once the opening reaches 180 degrees, it is no longer obtuse; it has become a straight angle.

Look at the two rays rather than the page direction. An obtuse angle may point down, lean sideways, or appear inside a rotated shape. Compare its measure with 90 and 180, and the classification will stay dependable.

• 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 obtuse angles appear

• 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.

Three obtuse-angle checks

• 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.