Put formulas on the left position in LateX

Question

Possible Duplicate:
Left align (to the page) displayed math

As you know the following equation will be in the center:

$$
\sum_{\substack{a\leq1/2\\0<b\leq T}}\frac{2+a}{2|b|}
$$

And the following is on the left but in small size and decrease the space:

$
\sum_{\substack{a\leq1/2\\0<b\leq T}}\frac{2+a}{2|b|}
$

But the below formula is similar to the first one, but on the left:

$
\displaystyle{\sum_{\substack{a\leq1/2\\0<b\leq T}}\frac{2+a}{2|b|}}
$

Does anyone know another way such that it works similar to the last one (put the formula on the left) but with:

\begin{xxxx}
*
\end{xxxx}

Answer 1

You can use the flalign* environment from amsmath to put equations flush with the left hand side:

\documentclass{article}
\usepackage{showframe}% http://ctan.org/pkg/showframe
\usepackage{amsmath}% http://ctan.org/pkg/amsmath
\begin{document}
\begin{flalign*}
  \sum_{\substack{a\leq1/2\\0<b\leq T}}\frac{2+a}{2|b|} &&
\end{flalign*}
\end{document}​

The extra alignment characters && is required. The showframe is merely to highlight the text block boundary and not needed in your final document.

Answer 2

By passing the fleqn option to your document class, you can achieve that display style math is moved to the left instead of being centered.

\documentclass[fleqn]{article}

\begin{document}
    \[ \sum_{a\leq1/2}\frac{2+a}{2|b|} \]
\end{document}