In △ ABC, a sin (B - C) + b sin (C - A) + c sin (A-B) =
[ISM Dhanbad 1973]
0
a sin (B-C) + b sin(C-A)+c sin(A-B)
= k(∑ A sin(B - C) = k{∑ sin(B + C) sin(B - C)}
=k={∑12(cos 2C−cos 2B)}=0.
Note: Students should note here that most of the expressions containing the cyclic factor associating with ‘–’ reduces to 0.