ข้อ 9
จาก
$\begin{array}{rcl}
(\sin\frac{\pi}{6})^{4x}&=&(\sqrt[5]{2^{30}})(\sec^2\frac{\pi}{3})\\
\left(\frac{1}{2} \right)^{4x} &=& 2^6.2^2\\
2^{-4x} &=& 2^8\\
-4x &=& 8\\
x &=& -2
\end{array}$
ดังนั้น $x^3-4x=(-2)^3-4(-2)=0$
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