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A triangle is a 2D shape with three sides and three angles.

The angles in a triangle add up to 180°.

The properties of different kinds of triangles are listed below.

Equilateral triangle

• all sides are of equal length - shown by the dashes on the lines
• all angles are equal (60°)

Isosceles triangle

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

Scalene triangle

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

Right-angled triangle

• contains a right angle 90°
• this means the other two angles add up to 90°
• a right-angled triangle can be either isosceles or scalene

Acute-angled triangle

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

Obtuse-angled triangle

• contains an obtuse angle
• can be either isosceles or scalene

Within all triangles:

• the longest side is opposite the largest angle
• 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