[tex]\bf 15tan^3(x)=5tan(x)\implies 3tan^3(x)=tan(x)
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3tan^3(x)-tan(x)=0\implies tan(x)[3tan^2(x)-1]=0
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\begin{cases}
tan(x)=0\\\\
\measuredangle x=0,\pi \\
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3tan^2(x)-1=0\\
tan^2(x)=\frac{1}{3}\\
tan(x)=\pm\sqrt{\frac{1}{3}}\\\\
tan(x)=\pm\frac{1}{\sqrt{3}}\\\\
\measuredangle x=\frac{\pi }{6}, \frac{5\pi }{6},\frac{7\pi }{6}, \frac{11\pi }{6}
\end{cases}[/tex]
