acute angle An angle that measures less than 90 degrees. right angle An angle that measures exactly 90 degrees. obtuse angle An angle that measures betwwen 90 and 180 degrees. straight angle An angle that measures exactly 180 degrees. An angle that measures exactly 180 degrees. Angles whose measure adds up to 90 degrees.
An acute triangle is a triangle with all three angles acute (less than 90°). An obtuse triangle is one with one obtuse angle (greater than 90°) and two acute angles. Since a triangle's angles must sum to 180°, no triangle can have more than one obtuse angle.
In geometry, an equilateral triangle is a triangle in which all three sides are equal. In the familiar Euclidean geometry, equilateral triangles are also equiangular; that is, all three internal angles are also congruent to each other and are each 60°.