Step-by-step explanation:
See the attached image. I numbered angles differently so as not to make an assumption that angles numbered the same are congruent.
[tex]\overline{AB} \parallel \overline{CD}\\\angle1 \cong \angle3[/tex] (alternate interior angles are congruent)
[tex]\overline{AD} \parallel \overline{BC}\\\angle 2 \cong \angle 4[/tex] (alternate interior angles are congruent)
[tex]\overline{AB} \cong \overline{CD}[/tex] (opposite sides of a parallelogram are congruent)
[tex]\triangle{ABO \cong \triangle{CDO}[/tex] (ASA - side/angle/side)
[tex]\overline{AO} \cong \overline{CO}\\\overline{BO} \cong \overline{DO}[/tex] (corresponding parts of congruent triangles are congruent)
The diagonals bisect each other!