# 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 |