tes

\begin{aligned} \log_b (x) &= \log _b \left( c \right)\log _c \left( x \right) \\ &= \frac{{\log _c \left( x \right)}}{{\log _c \left( b \right)}}  \end{aligned}

\begin{aligned}\boxed {\bar x = \frac{1}{n} \sum \limits_{i=1}^{n} {x_i}}

$\displaystyle \bar{x}=\frac{1}{n}\sum\limits_{i=1}^{n}{x_i}$ \end{aligned