Nữa mn sin(a + b + c) + sin(b + c – a) + sin(c + a – b) + sin(a + b – c)

sin(a + b + c) + sin(b + c – a) + sin(c + a – b) + sin(a + b – c)

= (sin(a + b + c) + sin(b + c – a)) + (sin(c + a – b) + sin(a + b – c))

= 2 × sin((a + b + c + b + c – a)/2) × cos((a + b + c – b – c + a)/2) + 2 × sin((c + a – b + a + b – c)/2) × cos((c + a – b – a – b + c)/2)

= 2 × sin((2b + 2c)/2) × cos(2a/2) + 2 × sin(2a/2) × cos((2c -2b)/2)

= 2 × sin(b + c) × cosa + 2 × sina × cos(c – b)

= 2 × (sin(b + c) × cosa + cos(c – b) × sina)

GOOD LUCK!