What makes an angle acute

An acute angle measures more than 0 degrees and less than 90 degrees. Ninety degrees is a right angle, so it is not acute. The size of the opening, rather than the length of the drawn sides, determines the classification.

Meaning and context

For example, 25°, 45°, and 89° are acute. An angle of 90° belongs to the right-angle category, and an angle of 120° is obtuse. Use the numerical measure when it is supplied instead of judging only by the drawing.

Examples and usage

If an angle is given in radians, the acute range is between 0 and π/2. The inequalities are strict: neither endpoint is included. A protractor or a calculation can help confirm a diagram that is not drawn to scale.

What to check

An acute triangle is a separate classification: all three of its interior angles are acute. A right triangle still has two acute angles, but that does not make the whole triangle acute. Name the angle or figure you are classifying.

Explore more: Math and Measurement