Given:
mCD=40
CA=BD
Since O is the center of the circle, CO, OA, OB and OD are the radii of the circle.
Hence, CO=OA=OB=OD.
So, the corresponding sides of traingle ACO and triangle DBO are equal.
Thus, according to SSS theorem, the triangles ACO and DBO are congruent.
Therefore, the angles opposite to corrsponding equal sides of the triangles are equal.
Hence, Also, Since the angle intercepted by arc AB and CD are equal,
mCD =mAB=40.
Therefore, mAB=40.