## Elementary Geometry for College Students (7th Edition)

1. $f$ and $g$ intersect. 2. Both $\angle ACD$ and $\angle BCE$ are straight angles. 3. $m\angle ACD = m\angle ACE +m\angle ECD = 180^{\circ}$ $m\angle BCE = m\angle ACE +m\angle ACB = 180^{\circ}$ 4. $m\angle ACE +m\angle ECD = m\angle ACE +m\angle ACB$ 5. $m\angle ECD = m\angle ACB$ 6. If two angles have the same angle measure, they are congruent. Therefore $\angle ECD$ and $\angle ACB$, which are vertical angles, are congruent.