Prove that the diagonals of a parallelogram bisect each other.
Let ABCD be the parallelogram. Therefore, AB || DC and AD||BC.
Consider triangle AOD and COB.
AD = BC (opposite sides of a parallelogram)
∠DAO = ∠BCO (Alterante angles)
∠ADO = ∠CBO (Alternate angles)
Therefore, by ASA congruency, the triangle are congruent.
Now AO = OC and BO = OD because they are corresponding sides of two congruent triangle. Thus, the diagonals of a parallelogram bisect each other.