There are three environments that put LaTeX in math mode: math, displaymath, and equation. The math environment is for formulas that appear right in the text. The displaymath environment is for formulas that appear on their own line. The equation environment is the same as the displaymath environment except that it adds an equation number in the right margin.
The math environment can be used in both paragraph and LR mode, but the displaymath and equation environments can be used only in paragraph mode. The math and displaymath environments are used so often that they have the following short forms:
\(...\) instead of \begin{math}...\end{math}
\[...\] instead of \begin{displaymath}...\end{displaymath}
In fact, the math environment is so common that it has an even shorter form:
$ ... $ instead of \(...\)