Properties of triangles

The angles in a triangle add up to {180}^\circ.

The properties of different kinds of triangles are listed below.

Equilateral triangle

  • all sides are of equal length
  • all angles are equal ( {60}^\circ)
Equilateral triangle diagram

Isosceles triangle

  • two sides are of equal length
  • two angles are equal (called the base angles)
Isosceles triangle diagram

Scalene triangle

  • the three sides are all different lengths
  • none of the angles are equal
Scalene triangle diagram

Right-angled triangle

  • contains a right angle
  • can be either isosceles or scalene
Right angled triangle

Acute-angled triangle

  • all three angles are acute
  • can be equilateral, isosceles or scalene
Acute angled triangle

Obtuse-angled triangle

Obtuse angled triangle
curriculum-key-fact
Within all triangles, the longest side is opposite the largest angle, and the shortest side is opposite the smallest angle. If two, or three, angles are the same size, then the sides opposite are of equal length.