\begin{eqnarray*}
& & \frac{3}{4 \pi}   \sqrt{4 \cdot x^2   12}\\
& & \lim_{n \to \infty}
  \sum_{k=1}^n \frac{1}{k^2} = \frac{\pi^2}{6}\\
& & {\it f}(x) = \frac{1}{\sqrt{x} x^2}\\
& & e^{i \pi} + 1 = 0\;
\end{eqnarray*}