Nilai cos 105 = k Nilai k adalah?

k = cos105°
k = cos(60+45)°
k = cos60cos45 - sin60sin45
k = 1/2 × 1/2 akar2 - 1/2 akar3 × 1/2 akar2
k = 1/4 akar2 - 1/4 akar6
k = 1/4 ( akar2 - akar6)