Nilai cos 7/12phi+cos phi/12

btw yang bener pi, bukan phi

[tex]\cos{\frac{7\pi}{12}}+\cos{\frac{\pi}{12}}=2\cos{\left(\frac{1}{2}\left(\frac{7\pi}{12}+\frac{\pi}{12}\right)\right)}\cos{\left(\frac{1}{2}\left(\frac{7\pi}{12}-\frac{\pi}{12}\right)\right)}\\=2\cos{\frac{\pi}{3}}\cos{\frac{\pi}{4}}\\=2\times \frac{1}{2}\times \frac{\sqrt{2}}{2}\\=\frac{\sqrt{2}}{2}[/tex]

cos 105 + cos 15 = 2 cos (105+15)/2 cos (105-15)/2
= 2 cos 60 cos 45
= 2 1/2 cos 1/2 akar 2
= 1/2 akar 2