A triangle is a polygon, that is, a flat geometric figure consisting of three sides, three vertices and three angles, which add up to 180º. Triangles are classified according to their characteristics, that is, according to the size of their sides and the amplitude of their angles.
The names of the triangles according to their sides are: equilateral, isosceles and scalene. Each of them has different characteristics that we will develop below.
The equilateral triangle is one that is characterized by having all equal sides. Consequently, all the angles of an equilateral triangle are 60º. The equilateral triangle is a regular polygon.
Isosceles triangles are characterized by having two equal sides and one different side. Consequently, it also has two equal angles.
A scalene triangle is one that is characterized by having all its sides and angles unequal, that is, different from each other.
Triangles can be classified according to the amplitude of their angles, which can be straight (equal to 90º); acute (less than 90º) and obtuse (greater than 90º).
Right triangles are those that are formed by a right angle and two acute angles. Therefore, the longest side is the hypotenuse.
For example, some isosceles and scalene triangles. That, however, can never happen with an equilateral triangle since the measure of its angles is invariable.
Oblique triangles are those that are characterized by not having any right angles. In this group are both acute and obtuse angles that, although they are different from each other, share this characteristic.