nilai dari perkalian cos dan sin : 2cos 120derajat cos 15derajat

2 cos 120° cos 15°
= 2 (-1/2) cos 15°
= -1 cos (45° - 30°)
= -1 (cos 45° cos 30° + sin 45° sin 30°)
= -1 (1/2 √2 . 1/2 √3 + 1/2 √2 . 1/2)
= -1 (1/4 √6 + 1/4 √2)

= -1/4 (√6 + √2)

= -1/4 √2 (√3 + 1)

= -1/4 √6 - 1/4 √2