\documentclass[12pt]{article}
 \textheight22cm \textwidth11cm \parindent0mm \parskip 1ex plus2ex minus0.8ex
\begin{document}
$\log ab=\log a+\log b$                  \\[+2ex]  
${\rm fun\,}ab={\rm fun\,}a+\ldots$      \\[+2ex] 
${\cal F}[f(x)]=F(u)$                    \\[+2ex]
$x^5$                                    \\[+2ex]
$x_1$                                    \\[+2ex]
$\sqrt{x^2+\sqrt[3]{y}}$                 \\[+2ex]
$\frac{1}{\frac{x^2+y^2+z^2}{x+y}}$      \\[+2ex]
$\frac{1}{\frac{x^2+y^2+z^2}{x+y}}$      \\[+2ex]
${n}\choose{n-k}$                        \\[+2ex]
$\int_{-\infty}^{\infty}x^3 dx$          \\[+2ex]
$\int\limits_{-\infty}^{\infty}x^3\,dx$  \\[+2ex]
$\sum_{i=1}^{n}a_i$                      \\[+2ex]
$\left( \int\limits_{a}^{b} (x+1)(x-1)\,dx \right)^{2}$      \\[+2ex]
\begin{displaymath}
  x = \cases    {
    \begin{array}{ll} y   & \mbox{\Large if $y>0$} \\
                      z+y & \mbox{\Large otherwise}
    \end{array} }
\end{displaymath}
$\vec{a}=\vec{\imath},\;\vec{\jmath}\neq\tilde{\jmath}$      \\[+2ex]
$\overline{ \overline{x}^{2}+1 }$                            \\[+2ex]
The \underline{value} is $\underline{4x}$                    \\[+2ex]
$\overbrace{a+\underbrace{b+c}_{26}+d}^{24}$                 \\[+2ex]
$\stackrel{a'}{\rightarrow}B\stackrel{\rm def}{=}
  \left(x_1,\ldots,x_n\right)$                               \\[+2ex]
\end{document}
