Triangle: literally means three (tri) angles and so has 3 interior angles. But triangles are very special due to the wide variety of types of their properties.
Triangles have interior angles which sum to 180°. But why?
Acute triangles
An acute triangle has interior angles that are all acute (<90°).
Right angle
A right angle triangle has one angle equal to 90°
Obtuse triangles
An obtuse triangle has one obtuse angle.
Equilateral triangles
An equilateral triangle has three sides of equal length, and therefore all three interior angles are equal 60°.
Isosceles triangle
The word isosceles literally translates as 'equal legs'. An isosceles triangle is a triangle with at least two equal side lengths and to equal angles.
Isosceles right angle
Like an isosceles triangle but with a right angle. Two sides remain the same length and two angles are equivalent.
Scalene
A scalene triangle has unequal sides and angles.
