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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:

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

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!!