nilai 2cos 150 cos 15

Trigonometri.
Matematika peminatan kelas XI kurikulum 2013 revisi 2016.

cos (α - β) = cos α cos β + sin α sin β
cos 15° = cos (45° - 30°)
= cos 45° cos 30° + sin 45° sin 30°
= 1/2 √2 (1/2 √3) + 1/2 √2 (1/2) = 1/4 (√6 + √2)

2 cos 150° cos 15°
= 2 (-1/2 √3) [1/4 (√6 + √2)]
= -1/4 (3√2 + √6)