tg x + tg 3 x = 0 помогите

[tex]tgx+tg3x=0\\tgx+\frac{3tgx-tg^3x}{1-tg^2x}=0\\\frac{tgx-tg^3x+3tgx-tg^3x}{1-tg^2x}=0\\\frac{4tgx-4tg^3x}{1-tg^2x}=0\\\frac{4tgx(1-tg^2x)}{1-tg^2x}=0\\4tgx=0\\tgx=0\\x=\pi k,\;k\in\mathbb{Z}[/tex]