Tìm $x$ để $\overrightarrow{a}\perp\overrightarrow{b},$ biết:
a) $\overrightarrow{a}=\left(\sin x;\,\sqrt{3};\,-4\tan^3x\right),\,\,\overrightarrow{b}=\left(3;\,-\dfrac{1}{2};\,\cos^3x\right)$
b) $\overrightarrow{a}=\left(\log_23;\,4\log_23;\,\log_2\left(\sqrt{2}-1\right)\right),\,\overrightarrow{b}=\left(\log_3x;\,\log_3\dfrac{1}{4};\,-\log_{\left(\sqrt{2}+1\right)}x\right)$