Mathematical terms have to be put either between
\( and \) or
between $ and $ or
between \begin{math} and \end{math}
. Mathematical terms are complete mathematical formula as well as single
variables correlated to formulas, Greek letters or special symbols. Since
LaTeX is mainly created for mathematical texts, the following examples
gives an impression of how LaTeX can look like when it's used in the right way.
Seien a und b die Katheten
und c die Hypotenuse,
dann gilt 
(Pythagor"aischer Lehrsatz).
Seien $a$ und $b$ die Katheten
und $c$ die Hypotenuse,
dann gilt $c^{2}=a^{2}+b^{2}$
(Pythagor"aischer Lehrsatz).
TeXspricht man wie
aus.[6pt]
100 m 
Nutzfl"ache [6pt]
Mit 
-lichen
Gr"u"sen
\TeX\ spricht man wie
$\tau\epsilon\chi$ aus.\\
100~m$^{2}$ Nutzfl"ache \\
Mit $\heartsuit$-lichen
Gr"u"sen
Longer mathematical formulas or equations should be but in a separate
line. For this purpose one should pose them between
\[ und \]
or between \begin{displaymath} and \end{displaymath}
or between \begin{equation} and \end{equation},
if you want the equations to be enumerated.
Seien a und b die Katheten und c die Hypotenuse, dann gilt
(Pythagor"aischer Lehrsatz).
Seien $a$ und $b$ die Katheten
und $c$ die Hypotenuse,
dann gilt
\begin{equation}
c = \sqrt{ a^{2}+b^{2} }
\end{equation}
(Pythagor"aischer Lehrsatz).