In figure O is the center of circle and Point A,B,C and D on circumference of circle .
Then OA, OB ,OC and OD is the radius of circle
In ΔAOB and COD
AB=AC (given)
OA=OC (Radius)
OB=OD (Radius)
∴ΔAOB≅COD
∴∠AOB=∠COD=900 Given (∠AOB=900)
Then (∠COD=900)