Definition
Acute triangle
An acute triangle is a triangle with all three interior angles less than 90°.
Continue from this idea into connected AXIO math.
What it means
Check every corner, not just the sharpest-looking one. A triangle is acute only when all three inside angles are smaller than a right angle. Turning or flipping the picture does not change the angle measures, so orientation does not change the classification.
Remember it
All three corners must be smaller than a right angle.
Example
A triangle has angles 48°, 61°, and 71°. Each angle is below 90°, and 48 + 61 + 71 = 180, so the triangle is acute.
Watch for
A triangle with one 90° angle is right, and a triangle with one angle greater than 90° is obtuse; neither is acute.
Common mix-up
One acute angle is not enough—every triangle has at least two acute angles. All three angles must be acute for the triangle to be acute.
Exact meaning
An acute triangle is a non-degenerate triangle in which each of the three interior angles is acute: every angle is greater than 0° and less than 90°.