hitunglah cos²15 + cos²75

jawab

cos² 15 + cos² 75 =

= cos² 75 + cos² 15
= (cos 75 + cos 15)² - 2 cos 75 cos 15
= {2 cos 1/2 (75+15) cos 1/2 (75-15)} - {cos (75+15) +cos (75-15)}
= (2 cos 45 cos 30) - (cos 90 + cos 60)
= 2 (1/2 √2)(1/2 √3) - (0 + 1/2)
= 1/2 √6 - 1/2
= 1/2 (√6 - 1)
= 1/2 (- 1 + √6)