# The same height of text in a large document

I’m typesetting a large document that contains a lot of mathematics. The only problem is that the height of text on some pages that ends with a displayed formula is less than (about 3-6mm) the height of pages that ends normally (without displayed formula).
I want that the height of text on all of my pages is quite the same. Does anybody know how to solve this problem? Is there any command that I should use in those pages in order to solve the problem?
By the way, I’m using options a4paper, twoside and book class in my document.

-
Are you using \raggedbottom by any chance? Try to explicitly set \flushbottom very late in your preamble and see if it changes anything. Note that \flushbottom is the default for book. – Martin Scharrer Apr 7 '11 at 10:22
@Martin: I've tried both \raggedbottom and \flushbottom, but they don't change anything. BTW, what do you mean very late? – user4669 Apr 7 '11 at 11:41
With "very late" I meant at the end of the preamble. – Martin Scharrer Apr 7 '11 at 11:48
@Martin: thanks for your reply. I tried that, but it doesn't work. – user4669 Apr 7 '11 at 18:49
contrary to what Ian Thompson said, the \belowdisplayskip is removed if it falls at a page break, so you should try providing a minimal example. – egreg May 8 '11 at 9:43

I think this is caused by the fact that a vertical space is inserted after each displayed equation. The size of this space is controlled by \belowdisplayskip. Have a look at section 11.5.1 of Mathmode.pdf.
There were some discussions of \cleader in some recent questions, which might help a little bit. – Bruno Le Floch Apr 8 '11 at 5:09