1) sin 100° - sin 40° = 2sin 30° cos 70° = 2 0.5 * cos 70° = cos 70°
2) cos 3a + cos 11a = 2cos 7a * cos 4a
3) cos 2a + cos 8a = 2cos 5a * cos 3a
1) sin 100° - sin 40° = 2sin 30° cos 70° = 2 0.5 * cos 70° = cos 70°
2) cos 3a + cos 11a = 2cos 7a * cos 4a
3) cos 2a + cos 8a = 2cos 5a * cos 3a