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Tyler has written an incorrect proof to show that quadrilateral \(ABCD\) is a parallelogram. He knows segments \(AB\) and \(DC\) are congruent. He also knows angles \(ABC\) and \(ADC\) are congruent. Find the mistake in his proof.
Segment \(AC\) is congruent to itself, so triangle \(ABC\) is congruent to triangle \(ADC\) by Side-Angle-Side Triangle Congruence Theorem. Since the triangles are congruent, so are the corresponding parts, and angle \(DAC\) is congruent to \(ACB\). In quadrilateral \(ABCD\), \(AB\) is congruent to \(CD\), and \(AD\) is parallel to \(CB\). Since \(AD\) is parallel to \(CB\), alternate interior angles \(DAC\) and \(BCA\) are congruent. Since alternate interior angles are congruent, \(AB\) must be parallel to \(CD\). Quadrilateral \(ABCD\) must be a parallelogram since both pairs of opposite sides are parallel.