Применим формулу:

cosa + cosb = 2cos((a+b)/2) • cos(a-b)/2)

В самом задании получится:

cos(π/2) + cos(π/12) =

= 2cos((π/2+π/12)/2) • cos((π/2-π/12)/2) =

= 2cos(7π/24) • cos(5π/24)