## Elementary Geometry for College Students (6th Edition)

1. It is given that angle CDE is supplementary to angle 1. Thus, m $\angle$ CDE = 180 - m $\angle$ 1 2. Angle ABC + m $\angle$ 3+ m $\angle$ 2 = 180 degrees since the shape is a triangle. 3. Thus, m $\angle$ ABC = 180 - m $\angle$ 2- m $\angle$ 3. 4. Since the polygon is regular, m $\angle$ ABC = m $\angle$ CDE 5. Using substitution, we see that m $\angle$ 1 = m $\angle$ 2 + m $\angle$ 3.