In any triangle ABC,
a/sin A = b/sin B = c/sin C = k
a = k sin A, b = k sin B, c = k sin C
LHS
= a sin (B C) + b sin (C A) + c sin (A B)
= k sin A [sin B cos C cos B sin C] + k sin B [sin C cos A cos C sin A] + k sin C [sin A cos B cos A sin B]
= k sin A sin B cos C k sin A cos B sin C + k sin B sin C cos A k sin B cos C sin A + k sin C sin A cos B k sin C cos A sin B
= 0
= RHS
Hence proved that a sin (B C) + b sin (C A) + c sin (A B) = 0.