Triangle Theorems

There are theorems (rules) that relate to triangles:

Triangle Angle Sum Theorem – the sum of the angles in a triangle is 180°

Triangle Side Sum (Inequality) Theorem – the sum of the lengths of any two sides of a triangle must EXCEED the length of the third side.

*If a, b and c are the lengths of the sides of a triangle and c is the largest side, then
(1) a²+ b² = c² => right triangle

(2) a² + b² > c² => acute-angled

(3) a²+ b² < c² => obtuse-angled

Exterior Angle of a Triangle