Dokaži da za svaki realni broj xxx vrijedi cos3x3+cos3x+2π3+cos3x+4π3=34cosx.\cos^3 \frac{x}{3} + \cos^3 \frac{x + 2\pi}{3} + \cos^3 \frac{x + 4\pi}{3} = \frac{3}{4} \cos x.cos33x+cos33x+2π+cos33x+4π=43cosx.