In △AOB and △OBC,
OA=OC (Given)
AB=BC (Given)
OB=OB (COmmon)
Thus, △AOB≅△COB (SSS rule)
Hence, ∠AOB=∠COB=x (By cpct)
We know, ∠AOB+∠COB=180 (Angles on a straight line)
x+x=180
x=90∘
Thus, ∠AOB=∠COB=90∘
∠AOB=∠=COD=90∘ (Vertically opposite angles) (I)
∠COB=∠=AOD=90∘ (Vertically opposite angles) (II)
Thus, from I and II
∠COD=∠AOD ..(III)
Now, In △AOD and △COD,
AO=OC (Given)
∠AOD=∠COD (From III)
OD=OD (Common)
thus, △AOD≅△COD (SAS rule)
Hence, AD=CD (By cpct)