Jawab:
trigonometri
cos (180+ a) = – cos a
cos (180- a) = - cos a
nilai
cos 105 + cos 195 - cos 15 +cos 75 =
= cos (180-75) + cos (180+15) – cos 15 + cos 75
= – cos 75 – cos 15 – cos 15 + cos 75
= – 2 cos 15
= -2 cos (45 – 30)
= – 2 { cos 45 cos 30 + sin 45 sin 30}
= -2 { 1/2 √2 . 1/2√3 + 1/2√2 .1/2)
= – 2 { 1/4 √6 – 1/4 √2)
= – 2 (1/4)(√6 – √2)
= -1/2 (√6 – √2)
