When you want to display math you should switch to math mode by enclosing the equations like the following:
\[
\frac{1}{q_2} = \frac{A q_{1} + B}{C q_{1} + D}
\]
OR
$$
\frac{1}{q_2} = \frac{A q_{1} + B}{C q_{1} + D}
$$
If you want have have math within a block of text, ie. in-line, use...