<span>cos5xcos4x = cos7xcos6x _____ </span><span>можно записать и так</span>
(cos(5x-4x) + cos(5x+4x))/2 = (cos(7x-6x) + cos(7x+6x))/2 _____ по формуле для произведения функций
cos(x) + cos(9x) = cos(x) + cos(13x)
cos(13x) = cos(9x)
13x = 9x + 2пK _____ 13x = п - 9x + 2пN
4x = 2пK _____ 22x = п(2N+1)
x = Kп/2 _____ x = (2N+1)п/22
N,K принадлежат Z