Given :
In circle C (O, r) chord AB = chord CD
and OM ⊥ AB and ON ⊥ CD
To prove : ∠OMN = ∠ONM
Proof : We know that equal chord of a circle are equidistant from center.
Thus OM = ON ΔOMN will be an isosceles triangle.
In isosceles triangle angles opposite to equal sides are equal
Thus ∠OMN = ∠ONM