a) cos п/12*cos п/4 = 0.5(cos(п/4-п/12) + cos(п/4+п/12)) = 0.5(cosп/6 + cosп/3)=0.5(√3/2 + 1/2)=(√3+1)/4
б) sin 15° * sin 75° = sin 15° * cos 15° = 0.5*2sin 15° * cos 15° = 0.5*sin30° = 0.25
в) cos 75° * cos 105°= sin 15° * (-sin 15°) = -sin²15° = -0.5*2sin²15° = -0.5*(1-cos30°) = -0.5*(1-√3/*2) = (√3-2)/4