A diagonal of a parallelogram forms two congruent triangles.

1.     Parallelogram ABCD with diagonal BD.                1.     Given.

2.     /  ABD = /  BDC.                                                 2.     Alternate Interior Angles are                                                                                              Equal.

3.     /  ADB = /  DBC.                                                 3.     Alternate Interior Angles are                                                                                              Equal.

4.     DB = DB.                                                             4.     Reflexive Property.

5.     Triangle ADB = Triangle CBD.                          5.     ASA.

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