Lesson 2 of 8
Triangles
A triangle is a three-sided shape, and it shows up everywhere in geometry. You sort triangles two ways: by their sides and by their angles.
By sides
- ●Equilateral: all three sides equal (and all angles 60 degrees).
- ●Isosceles: two sides equal.
- ●Scalene: no sides equal.
By angles
- ●Acute: all angles less than 90 degrees.
- ●Right: one angle exactly 90 degrees.
- ●Obtuse: one angle more than 90 degrees.
FORMULA
The three angles inside any triangle always add up to 180 degrees.
FORMULA
Triangle inequality: the two shorter sides added together must be greater than the longest side, or it cannot form a triangle.
Example: if two angles are 50 degrees and 60 degrees, the third is 180 - 50 - 60 = 70 degrees. Sides of 3, 4, and 5 work because 3 + 4 = 7 is greater than 5.