tgx-2tgx/(1-tg^2x)=(tg^3x-tgx)/(1-tg^2x)=tgx(tg^2x-1)/(1-tg^2x)=-tgx
tgX - 3ctgX = 2
tgX - 3/tgX = 2
tg²X - 3 = 2tgx
tg²X - 2tgX - 3 = 0
(tgX - 3)(tgX +1) = 0
tgX = 3 ili tgX = -1
X₁ = arctg3 X₂ = 135