Here is the completed proof with the missing statements and reasons filled in:
Statements | Reasons --- | ---
- Isosceles trapezoid EFGH with \( EF \cong GH \) | 1. Given
- \( \angle EHG \cong \angle FEH \) | 2. Base angles of isosceles trapezoid are congruent
- \( m\angle EHG + m\angle FGH = 180^\circ \) | 3. Same side interior angles are supplementary
- \( m\angle EHG + m\angle FGH = 180^\circ \) | 4. As stated in step 3
- \( m\angle EFG + m\angle EHG = 180^\circ \) | 5. Substitution Property
- \( \angle EFG \) and \( \angle EHG \) are supplementary. | 6. Definition of supplementary angles
By filling in the missing statements and reasons, we successfully prove that the opposite angles \( \angle EFG \) and \( \angle EHG \) are supplementary.