tg(α + β) =(tgα + tgβ)/(1 - tgαtgβ)
tg6x + tg3x = tg(6x + 3x)(1 - tg6xtg3x) = tg9x(1 - tg6xtg3x)
tg6x + tg3x - tg9x = 0
tg9x(1 - tg6xtg3x) - tg9x = 0
tg9x(1 - tg6xtg3x - 1) = 0
tg9xtg6xtg3x = 0
tg9x = 0 или tg6xtg3x = 0
9x = πk или 6x = πk или 3x = πk, k∈Z
x = πk/9 или x = πk/6 или x = πk/3
