cosA + cosB = 2 cos((A+B)/2) cos((A-B)/2)
y = cos(2x) + cos(2x-π/6）= 2 cos(2x -π/12 ) cos(π/12) =2 cos(15°) cos(2x -π/12 )
max：2 cos(15°) = (√6 + √2) / 2
min：- 2 cos(15°)