Justify informally that the interior angle sum of a triangle is 180 degrees, and that any exterior angle equals the sum of the two interior opposite angles

1. Triangle Tangle - Determine the size of an angle in a triangle given an algebraic expression for the relationship between the 3 angles
2. Tri-Trip - angle sum of a triangle
3. Tri-trip Terry - angle sum of a triangle
4. Angling With Ants - calculate angle size, angle sum of a triangle
5. Phoebe's Triangle Theorems 3 - equal angles opposite equal sides
6. Phoebe's Triangle Theorems 2 - exterior angle of triangle
7. Phoebe's Triangle Theorems 1 - angle sum of a triangle