Question 1089989
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If "< 1 & < 2 are supplementary" is given, then what immediately follows is that "m < 1 + m < 2 = 180" by the <font color=red>definition of supplementary angles</font>. In other words, if two angles are supplementary, then they add to 180 degrees. 


If m < 1 = m < 2, and m < 1 = 90, then m < 2 = 90 as well through the <font color=red>substitution property</font> (or you could argue the transitive property of equality which is very similar). You can think of it as starting off with m < 1 = m < 2, then replacing the "m < 1" with "90". This replacement is valid because of m < 1 = 90.
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