Exercise 37 Page 429

Let's begin by creating a linear pair.

The angles ∠ TSU and ∠ USV are a linear pair because they have a common side SU, and the non-common sides are opposite rays. They are also supplements of each other because the sum of their measures is 180^(∘). m∠ TSU+m∠ USV=120^(∘)+60^(∘)=180^(∘) When two angles are linear pairs the sum of their measures is always 180^(∘), and therefore they are by definition always supplementary.