you are here > Class Notes - Chapter 7 - Lesson 7-1

Printable version

Triangle Application Theorems - Lesson 7-1

Today, we finally proved that the sum of the angles of a triangle is 180 degrees!! This is clearly something you've know for quite a while (a couple of years at least!), but was something we couldn't prove until recently. With what we know about parallel lines and alternate interior angles, it'spretty straight forward:

Triangle Sum Theorem


We then formally defined exterior angles and showed that an exterior angle of a triangle equals the sum of the remote interior angles:

Definition of Exterior Angle


Exterior Angle Theorem


For the diagram above, the measure of angle 1 = measure of angle D + measure of angle E.

We wrapped up with the following theorem and its proof...very cool stuff here!!

Proof of the Midline Theorem

Other Links
Class Notes
Lesson 7-1
Lesson 7-2
Lesson 7-3
Lesson 7-4
If you have questions, email me at