A parallelogram with a diagonal drawn through it is shown below. Arlen is trying to prove that RS ≅ TU, but is unable to come up with one of the lines of the proof.

STATEMENTS REASONS

RSUT is a parallelogram Given

∠RST ≅ ∠UTS and ∠RTS ≅ ∠UST If parallel lines are cut by a transversal, alternate interior angles are congruent.

ST ≅ ST Reflexive Property

ΔRST ≅ΔUTS ???

RS ≅ TU Corresponding parts of a congruent triangle are congruent.

What is the missing reason in the proof?