join m and n to the centre of the circle.(point o)
so, omb=ond=90 [line joining centre of the circle and the centre of the chord is the perpendicular bisector of the chord]
omn=onm [om=on, and in an isoceles triangle, angles opposite to equal sides are equal]
so,
omb=ond and omn=onm
omb-omn=ond-onm
therefore, bmn=dnm.