Based on the △ AOD and △ BOC
From the figure we know that ∠ AOD and ∠ BOC are vertically opposite angles.
So we get
∠ AOD = ∠ BOC
We also know that
∠ DAO = ∠ CBO = 90o
It is given that AD = BC
Therefore, by AAS congruence criterion we get
△ AOD ≅ △ BOC
So we get AO = BO (c. p. c. t)
Therefore, it is proved that CD bisects AB.