Final answer:
To prove ABCD is a parallelogram, we can find two congruent triangles within the figure: triangle ADC and triangle BAC.
Step-by-step explanation:
To prove that ABCD is a parallelogram, we need to show that opposite sides are parallel and congruent. One way to do this is by finding two congruent triangles within the figure. In this case, we can use triangle ADC and triangle BAC. These triangles are congruent because they share side AC, which is congruent to itself, and they have the same angle at C. Since congruent triangles have congruent angles, we can conclude that ABCD is a parallelogram.