Question

ПОМОГИТЕЕ!!!!
ДОКАЖИТЕ ТОЖДЕСТВО
$cos a-cos(60 ^{circ} - a) - cos(60 ^{circ} + a)=0$

Answer 1

$cos alpha -cos(60^circ- alpha )-cos(60^circ+ alpha )=\\=cos alpha -(cos(60^circ- alpha )+cos(60^circ+ alpha ))=\\cos alpha -2cosfrac{60^circ- alpha +60^circ+ alpha }{2}cosfrac{60^circ- alpha -60^circ- alpha }{2}=\\=cos alpha -2cos60^circcos(- alpha )=cos alpha -2cos60^circcosalpha=\\cos alpha -2bulletfrac{1}{2}cos alpha=cos alpha -cos alpha =0.$