b sin B−c sin C=a sin (B−C)
sin Aa=sin Bb=sin Cc=kRHS,a sin (B−C)= a sin B.cos C − a sin C.cos B= a(bk).(a2+b2−c22ab)−a(ck).(a2+c2−b22ac)=k.(a2+b2−c2)2−k(a2+c2−b2)2=2k.(b2−c2)2=b.(kb)−c(kc)=b (sin B)−c (sin C)LHS.