Ctgx=1/tgxtg2x-1/tg5x=0, cos2x≠0, sin5x≠0 tg2x * tg5x -1 ------------------ =0 tg5x sin2x sin5x------- * -------- -1=0cos2x cos5x sin2x *sin5x-cos2x *cos5x =0, -(cos2x *cos5x-sin2x *sin5x)=0, -cos(2x+5x)=0, cos7x=0, 7x=π+2πn, x=π/7+2πn/7, n∈Z
надеюсь понятно?)