One of the main advantages of using LATEX is that it so good at typesetting
mathematics. Mathematical input in LATEX is typed in what is called
math mode. To type math that is to appear as part of a paragraph
or other piece of text, enclose the math input between a pair of dollar signs,
such as $x<y$. If you use a pair of double dollar signs,
$$x<y$$, you get display math mode where the mathematics
is displayed on a line by itself and centered horizontally on the page.
A careful mathematical typist will use math mode even for a thing
as simple as a one-character variable. This is because a variable such
as 
should properly be printed in italics. Math mode will take care
of this. This means that you should type the variable as $x$ rather
than simply as x.