Proofs...ugh.

[Roger]

How do I do this problem?

Theorem 2-11: The sum of the measures of the angles of a triangle is 180.

Problem: Prove Theorem 2-11 by using the figure below:

(sorry for bad drawing)

Please walk me through this. I'm getting a D- in Geometry right now.

Thanks to everyone that helps.

 

[Roger]

Problem solved.

 

[Gene]

Its not a bad drawing except there should be an E at the end of AC.
AB is parallel to CD.
They intersect AE at equal angles BAC and DCE.
ABC and BCD are alternate interior angles and so they are equal.
ACB is the third angle.
ACB + BCD + DCE add up to a st. line = 180° so
ACB + ABC + BAC = 180°