Tính :
$a)$ ${\log _a}\left(
{\frac{{{a^2}\sqrt[3]{{{a^2}}}.a.\sqrt[5]{{{a^4}}}}}{{\sqrt[3]{a}}}} \right)$
$b)$ ${\log _a}\left( {a\sqrt {a\sqrt {a\sqrt a } } } \right)$
$c)$ ${\log _a}\sqrt {\frac{1}{5}} .{\log _{25}}\sqrt[3]{2}$
$d)$ $\frac{{{{log }_4}64}}{{{{log }_2}64}}$
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