cos 195 derajat=1/4 √2 (1+√3) cos 225 derajat=1/4√2 (1-√3)

Nilai fungsi trigonometri sudut relasi

cos (195 ) =  cos (180 + 15)= - cos 15
= - cos ( 45- 30)
=- { cos 45 cos 30 + sin 45 sin 30}
= - {1/2 √2 . 1/2 √3  + 1/2 √2  . 1/2)
= - {1/4 √6 + 1/4 √2)
= - 1/4 √2 ( √3 + 1)
= -1/4 √2 ( 1 + √3)

cos 225 = cos (180 + 45) = - cos 45
= - 1/2 √2
= -1/4 √2 ( 2)