Answer
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.
