An acute triangle is a triangle where all three internal angles are acute (less than degrees).
In any triangle, two of the interior angles are always acute (less than degrees)*, so there are three possibilities for the third angle:
Less than ° - all three angles are acute and so the triangle is acute.
Exactly° - it is a right triangle
Greater than ° (obtuse): the triangle is an obtuse triangle
if the measurements of angles are:, and , than
so, and
since or greater than °, a triangle cannot be acute, the triangle is an obtuse triangle