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

1. $\angle 1$ is supplementary to $\angle 2$ $\angle 3$ is supplementary to $\angle 2$ 2. $m\angle1 +m\angle2 =180^{\circ}$ $m\angle3 +m\angle2 =180^{\circ}$ 3. $m\angle 1 +m\angle2 =m\angle3 + m\angle 2$ 4. $m\angle 1=m\angle3$ ( By subtracting $m\angle 2$ from both sides) 5. If two angles have the same angle measure, they are congruent. Therefore $\angle 1 \cong \angle 3$