cos5x*cos7x-cosx+sin5x*sin7x
cos5x*cos7x+sin5x*sin7x = cos(7x-5x) = cos2x - просто формула
cos2x-cosx = 2cos^2x-1-cosx
cos5x*cos7x-cosx+sin5x*sin7x
cos5x*cos7x+sin5x*sin7x = cos(7x-5x) = cos2x - просто формула
cos2x-cosx = 2cos^2x-1-cosx