Two types of triangle

There are three main types of triangle:

1. Scalene - all angles are different and all sides are different lengths
2. Isosceles - Two sides are equal in length and the two angles (between the third side and the two sides of equal length) are equal
3. Equilateral - All three angles are equal (at 60°) and all three sides are of equal length.

If all angles are less than 90° then the triangle can also be described as an Acute triangle; an equilateral triangle is acute by definition as all angles are 60° which is less than 90°.

A Scalene or Isosceles triangle can also be described as a Right angle triangle if one angle is 90° (a right angle); in the isosceles triangle this is the angle between the sides of equal length.

A Scalene or Isosceles triangle can also be described as an Obtuse triangle if one angle is greater than 90°; in the isosceles triangle this is the angle between the sides of equal length.