:o is the center
:ol and om are perpendicular to ab and cb respectively
consider tri.AOL and COM
hence,triangle AOL is congruent to tri. COM by aas congruence criteria
so, AL=CM (CPCT)
but, L and M are midpoints of ab and cb as OL and OM are the perpendiculars from the center and the perpendiculars from the center to the chord bisect the chord.
AL=LB & CM=MB
HENCE THE PROOF ...