What's so special about these angles, though? Well, one of the most important properties of triangles is the fact that the sum of the three inner angles of a triangle is always 180°. We call this little statement the **Angle Sum Theorem** for triangles.

We aren't gullible enough to believe everything we hear. Just because someone claims they saw some fairies doesn't mean we should buy out all the butterfly nets in the state to try and catch these mythical winged sprites. No, we'll need solid proof, and while we can't prove anything about fairies, elves, or leprechauns, we can fashion some sort of true statement about triangles and their angles. If seeing is believing, then the first thing we want to do is draw a picture of a triangle.

As pretty as it is, this picture doesn't tell us a whole lot. If we draw a line parallel to *AC* through the point *B*, that might tell us a bit more about what we're looking at. Let's label this parallel line segment *DE*.

We just learned a whole boatload of stuff about parallel lines, so this should help us. We know that alternate interior angles are congruent, so that means ∠*CAB* ≅ ∠*DBA* and that ∠*ACB* ≅ ∠*CBE*. So let's label that in our picture.

We can see that the total number of degrees in the triangle equals the number of degrees in ∠*DBE*. Since m∠*DBE* = 180°, we can say that the interior angles of any triangle will always add up to 180°.

Not convinced? We could write a formal proof to complete the Angle Sum Theorem with the triangle and a parallel line above it as our given data. But why should we do all the work?

Fill in the missing reasons and statements of the following proof that all interior angles of a triangle add up to 180°. The image below is our given data.

Statements | Reasons |

1. ∠CAB ≅ ∠DBA | 1. Alternate Interior Angle Theorem |

2. ∠ACB ≅ ∠CBE | 2. Alternate Interior Angle Theorem |

3. m∠DBA + m∠ABE = 180 | 3. ? |

4. m∠ABE = m∠ABC + m∠CBE | 4. ? |

5. ? | 5. Substitution Property (4 into 3) |

6. m∠CAB = m∠DBA | 6. Definition of Congruence (1) |

7. m∠ACB = m∠CBE | 7. Definition of Congruence (2) |

8. ? | 8. Substitution Property (6 and 7 into 5) |

9. Sum of interior angles is 180. | 9. Interpretation of 8 into words |

This may seem like a lot at first glance, but we can take it one step at a time. The first two steps are done for us, so let's move on to the third.

What allows us to say that m∠*DBA* and m∠*ABE* equal 180°? Well, they're supplementary angles. The fact that they add up to 180° is part of their definition. All we have to write is, "Definition of Supplementary Angles."

The next statement says that m∠*ABE* = m∠*ABC* + m∠*CBE*. Both ∠*ABC* and ∠*CBE* are adjacent, so we can add them up according to the Angle Addition Postulate.

Substituting the fourth statement into the third means we replace "m∠*ABE*" with "m∠*ABC* + m∠*CBE*." That means our statement is "m∠*DBA* + m∠*ABC* + m∠*CBE* = 180."

We can set the measures of congruent angles equal to each other according to the definition of congruence. Actually, we already did that. All that's left is to substitute those statements into the fifth statement. If we do that, we get "m∠*CAB* + m∠ABC + m∠*ACB* = 180." We can make the final claim because ∠*CAB*, ∠*ABC*, and ∠*ACB* are all the interior angles of the given triangle.

This theorem will be super-important to us. Like Proactiv, it'll work no matter how beautiful or how awkward our triangles are. The Angle Sum Theorem won't give us clear skin, but it will give us clearly defined angles.

Now that we know it isn't lying to us, we can use the Angle Sum Theorem to find the measures of some angles. Let's get to it!

A triangle has angles of 73° and 48°. What is the measurement of the final angle?

We know that all the angles in a triangle total to 180°, so we can set up the equation m∠1 + m∠2 + m∠3 = 180. Since we already know the measures of two angles, we can substitute them into the equation to find the measure of the third angle.

73 + 48 + m∠3 = 180

m∠3 = 59

That means the final angle in the triangle is 59°.

The angles inside of a triangle (those angles that have to add up to 180°) are called **interior angles**. 'Cause they're on the interior of the triangle. Duh.

Find the measure of *x*.

We can use the Angle Sum Theorem to find *x*, but for that to work, we have to find the other angles of the triangle. Luckily, we can do that.

One of the angles is already marked as 37°. One down; three to go.

We aren't given any other direct measurements, but another interior angle in the triangle has a supplementary angle of 113°. Since we know supplementary angles add up to 180°, the other angle has a measure of 180° – 113° = 67°.

All angles in a triangle add up to 180° (thanks, Angle Sum Theorem), we can add the angles up to find *x*.

37 + 67 + *x* = 180*x* = 76

We could calculate all the angles in a triangle, or we could use a special property about these things called exterior angles.

In this triangle, ∠*d* is the exterior angle. An **exterior angle** is what you get when you extend one of the sides of the triangle. The two angles not adjacent to an exterior angle, in this case ∠*a* and ∠*b*, are called **remote interior angles** (even though they're pretty close by).

Since an exterior angle and its adjacent interior angle are supplementary, they add up to 180°. According to the Angle Sum Theorem, angles ∠*a*, ∠*b*, and ∠*c* also add up to 180°. Since m∠*c* + m∠*d* = 180, and m∠*a* + m∠*b* + m∠*c* = 180, we could set them equal to each other.

m∠*c* + m∠*d* = m∠*a* + m∠*b* + m∠*c*

We have m∠*c* on both sides, so we can eliminate it.

m∠*d* = m∠*a* + m∠*b*

This is called the **Exterior Angle Theorem**, which says that the measure of an exterior angle is equal to the sum of the measures of the remote interior angles. It's useful just because it prevents us from having to do even more work.

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