cos(x-6分之π）=-3分之根3,则cosx+cos（x-3分之π）=

cosθ+cosφ = 2 cos[(θ+φ)/2] cos[(θ-φ)/2]cosx+cos（x-π/3）=2 cos[(x+x-π/3)/2] cos[(x-x+π/3)/2]=2 cos(x-π/6) cosπ/6=2(-3分之根3)(2分之根3)=-1