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Examine the following paragraph proof:

Consider quadrilateral ABCD, where AB is parallel to CD and BC is parallel to DA. Diagonal BD can be constructed.

- [tex]\(\angle ABD\)[/tex] is congruent to [tex]\(\angle CDB\)[/tex] because they are alternate interior angles.
- [tex]\(\angle BDA\)[/tex] is congruent to [tex]\(\angle DBC\)[/tex] because they are alternate interior angles.
- BD is congruent to itself by the reflexive property.

Therefore, [tex]\(\triangle ABD\)[/tex] and [tex]\(\triangle CDB\)[/tex] are congruent by ASA (Angle-Side-Angle). So, AB is congruent to CD and BC is congruent to DA because corresponding parts of congruent triangles are congruent.

What statement is proven by the steps above?

A. A quadrilateral with congruent opposite sides is a parallelogram.
B. The diagonals of a parallelogram are congruent.
C. The opposite sides of a parallelogram are congruent.
D. A quadrilateral with congruent diagonals is a parallelogram.

Answer :

We start by noting that quadrilateral [tex]\(ABCD\)[/tex] has one pair of opposite sides, [tex]\(AB\)[/tex] and [tex]\(CD\)[/tex], parallel, and the other pair, [tex]\(BC\)[/tex] and [tex]\(DA\)[/tex], also parallel. By drawing diagonal [tex]\(BD\)[/tex], two triangles—[tex]\(\triangle ABD\)[/tex] and [tex]\(\triangle CDB\)[/tex]—are formed.

1. In these triangles, the proof shows that two angles in [tex]\(\triangle ABD\)[/tex] are congruent to two angles in [tex]\(\triangle CDB\)[/tex] because they are alternate interior angles. That is,
[tex]$$ \angle ABD \cong \angle CDB \quad \text{and} \quad \angle BDA \cong \angle DBC. $$[/tex]

2. The diagonal [tex]\(BD\)[/tex] is a common side to both triangles, meaning
[tex]$$ BD \cong BD \quad (\text{reflexive property}). $$[/tex]

3. With two pairs of congruent angles and the common side, the ASA (Angle-Side-Angle) postulate confirms that
[tex]$$ \triangle ABD \cong \triangle CDB. $$[/tex]

4. By the CPCTC (Corresponding Parts of Congruent Triangles are Congruent) theorem, corresponding sides in the triangles are congruent. Hence, it follows that:
[tex]$$ AB \cong CD \quad \text{and} \quad BC \cong DA. $$[/tex]

This result shows that the opposite sides of quadrilateral [tex]\(ABCD\)[/tex] are congruent. Therefore, the statement established by the proof is:

[tex]$$\text{"The opposite sides of a parallelogram are congruent."}$$[/tex]

Thus, the correct answer is option 3.

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Rewritten by : Jeany