$\sum\limits_{n = 1}^n {{1 \over {{{\log }_{{2^n}}}(a)}}} = $
${{n(n + 1)} \over 2}{\log _a}2$
${{n(n + 1)} \over 2}{\log _2}a$
${{{{(n + 1)}^2}{n^2}} \over 4}{\log _2}a$
None of these
${\log _4}18$ is
If ${\log _k}x.\,{\log _5}k = {\log _x}5,k \ne 1,k > 0,$ then $x$ is equal to
$\log ab - \log |b| = $
${\log _7}{\log _7}\sqrt {7(\sqrt {7\sqrt 7 } )} = $
If ${a^x} = b,{b^y} = c,{c^z} = a,$ then value of $xyz$ is