Use angle facts and diagram structure to find unknown angle measures.
Use angle-pair totals
Problem
\(\text{One}~\text{angle}\) in a supplementary pair measures \(128°\). What is the measure of the other angle?
Big Picture
What this problem is really about
Begin with the relationship word, because it determines the total the two angles must make. Supplementary angles fill a straight angle, so represent the unknown as the part that completes that total after the known measure is included. Subtracting the known angle and then adding it back provides a direct check.
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