To complete the statements to prove that the sum of the interior angles of [tex]$\triangle ABC$[/tex] is [tex]$180^\circ$[/tex], follow these steps:
1. Points [tex]$A, B$[/tex], and [tex]$C$[/tex] form a triangle.
- Reason: Given.
2. Let [tex]$\overline{DE}$[/tex] be a line passing through [tex]$B$[/tex] and parallel to [tex]$\overline{AC}$[/tex].
- Reason: Definition of parallel lines.
3. [tex]$\angle 3 \simeq \angle 5$[/tex] and [tex]$\angle 1 \approx \angle 4$[/tex]
- Reason: Alternate interior angles formed by a transversal with parallel lines.
4. [tex]$m \angle 1 = m \angle 4$[/tex] and [tex]$m \angle 3 = m \angle 5$[/tex]
- Reason: Definition of congruent angles.
5. [tex]$m \angle 4 + m \angle 2 + m \angle 5 = 180^\circ$[/tex]
- Reason: Angle addition and definition of a straight line.
6. [tex]$m \angle 1 + m \angle 2 + m \angle 3 = 180^\circ$[/tex]
- Reason: Substitution.
Therefore, by substituting the angles, we have shown that the sum of the interior angles of [tex]$\triangle ABC$[/tex] is [tex]$180^\circ$[/tex].