Display-math after one tab distance from margin

Question

By default, latex beautifully places all display-math at the center of a line in the pdf file. I am preparing a document where I should start all display-math after one tab distance from the margin. The page will contain some nonmathematical text also. How can I do so?

The following example may clarify this question. Consider the following pdf-latex code:

\documenclass{article}

\usepackage{amsmath}

\begin{document}

There was a text like : aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa

$$(a + b)^2 = a^2 + 2ab + b^2$$

There is another text : bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb

\end{document}

Now in the pdf file we get $$(a + b)^2 = a^2 + 2ab + b^2$$ in the middle of a line. But I need it in the after one tab distance from the margin.