Question

Buktikan bahwa 2 sin 3x sin 4x + 2 cos 5x cos 2x - cos 3x = cos x

Answer 1

2 sin 3x sin 4x = - {cos (3x+4x) - cos (3x-4x)}
2 cos 5x cos 2x = cos (5x+2x) + cos (5x-2x)
=> - cos 7x + cos (-x) + cos 7x + cos 3x - cos x => cos (-x)
dimana cos (-x) = cos x (terbukti)