Jawabanmu

2014-01-05T23:39:54+07:00
Cos 135 + cos 75
= 2 cos 1/2 (135 + 75) cos 1/2 (135 - 75)
= 2 cos 1/2 x 210 cos 1/2 x 60
= 2 cos 105 cos 30
= 2 . (1/4 √2 - 1/4 √6) . 1/2 √3
= 1/4 √6 - 1/4 √18
= 1/4 √6 - 3/4 √2

kalo ngga salah..

btul22
tinggal dilanjut akar18 = 3 akar 2