cosx+cos2x+cos6+cos7x=4cos(x/2)*cos(5x/2)*cos4x докажите тождество
(cosx + cos7x) + (cos2x + cos6x) = 2cos4x*cos3x + 2cos4x*cos2x = 2cos4x*(cos3x+cos2x) = 2cos4x * 2cos(5x/2) * cos(x/2) = 4cos(x/2)*cos(5x/2)*cos4x