Question

cos11pi/12+cos5pi/12

Answer 1

$cos11pi/12+cos5pi/12=2cos frac{11pi/12+5pi/12}{2}*cos frac{11pi/12-5pi/12}{2}=\\=2cos16pi/24*cos6pi/24=\\=2cos2pi/3*cospi/4=2*(-1/2)*( sqrt{2}/2)=- sqrt{2}/2$