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Thank's a lot Stephan and cgnieder.

But I have found more convenient way to write these equations without changing my whole document.

I just used IEEEeqnarray instead of array
the IEEEeqnarray allow the pagebreaks,
here is the code:

\documentclass[12pt,a4paper]{article}
\usepackage{amsmath ...

Is there any way to toggle the left and right alignment of the "align".

The first column of "align" is right aligned the second is left and so on.
I want first column to left aligned then second is right and so on.

The second problem is I want to make newenvironment for this, so that I don't have ...

Thank's a lot Stefan_K
This is much better to use align instead of arrays.
But here a new question arises, I have to add a "&" at starting of all the equation I have written. And the document is about 250 pages, is there any other way to do so.

Here is the old code

\begin{array}{rrlc}
\because ...

here is my code:


\begin{array}{rrlr}
&& \sin{20\degree}\cdot\sin{40\degree}
\cdot\sin{60\degree}\cdot\sin{80\degree}\\
= && \frac{1}{2}\left[\cos{20\degree}-\cos{60\degree}
\right]\cdot\frac{\sqrt{3}}{2}\sin{80\degree}\\
= && \frac{\sqrt{3}}{4}\left[\cos{20\degree}
-\frac{1}{2}\right]\cdot ...

this command doesn't work with arrays.
I have tried this.
is there any other solution for this.
Or can I use other environment for writing these equation in more beautiful manner.

I am using array environment for mathematical derivations. Although this environment produce beautiful equation with perfect spacing but it also produce very much white space which is undesirable.
Is there any way to produce such derivation but allowing column and page breaks as well as the normal ...

When I use the mhchem package in my document, the text size is changing automatically. Plz help.