Question 907668
Basic Idea



<1 and <2 are supplementary  ... given

<2 and <3 are congruent ... given



<1 + <2 = 180 ... angles 1 and 2 are supplementary 

<2 = <3 ... since <2 and <3 are congruent


Since <1 + <2 = 180 and <2 = <3, we can replace the "<2" with "<3" to get <1 + <3 = 180. We are using the substitution property.


This isn't the formal proof, but it gives you an idea of how to do it.