Classifications by side length or comparison of angles and sides:
scalene - no sides or angles equal
isosceles - two sides and angles equal (congruent)
equilateral - all sides and angles equal (all 60° angles)
Classifications by included angles :
right triangle - contains a 90° angle (right angle)
oblique triangle - has no right angle
acute triangle - contains all angles less than 90°
obtuse triangle - contains one angle greater than 90°
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An octogon contains 6 equilateral triangles.
scalene triangles
6-2 = 4 triangles
8-2 = 6 triangles
Because all other polygons are made from triangles as for example an 8 sided octagon has 6 triangles within it or a 6 sided hexagon has 4 triangles within it.