English: Because m = 0, 0 that is 0, 0 divided by, here also sin0, 0 by 0, that is actually, what you have is b_(m) is anyways 0, b_(n).0, whatever, b_(n) maybe arbitrary constant.
Telugu: 0 బై 0, అంటే వాస్తవానికి, మీ వద్ద ఉన్నది b_(m) ఎలాగూ 0 అయినది, కావున ఇపుడు b_(n) కూడా .