There are six types of triangles based on the lengths of their sides and their angles. There are three triangles classified according to the length of their sides: equilateral, isosceles, and scalene. There are three triangles classified according to their angles: right, acute, and obtuse. A triangle belongs to only one category from each group. For example, a triangle can be acute and scalene, or right and isosceles.
Quick Triangle Review
To review, a triangle is a two-dimensional polygon with three sides, three vertices, and three interior angles. The sum of the three angles always adds up to 180°.
Three Types of Triangles Based on Sides
- Equilateral triangle: All three sides are the same length and all interior angles are 60°. This make an equilateral triangle a type of acute triangle.
- Isosceles triangle: Two sides of an isosceles triangle are the same length. The angles are the same at the base of both sides.
- Scalene triangle: None of the sides or interior angles of a scalene triangle are the same.
Three Types of Triangles Based on Angles
- Right triangle: One of the angles of a right triangle is 90° or a right angle. Because all three interior angles add up to 180°, the other two angles are acute. The long side of a right triangle is its hypotenuse.
- Acute triangle: All of the angles of an acute triangle are less than 90°.
- Obtuse triangle: One angle of an obtuse triangle is greater than 90°.