How do I break a long equation over lines?

I am trying to add an equation in a new line. The problem is that the equation is too long for the line, and I need to break it manually. Otherwise, it just overlaps to the right column, or to the right margins (and looks ugly...). Is there a way LaTeX can brake the equation for me, so it seems nice?

I'm attaching my latex code:

\begin{align*}
f(n)-f(0) &= A(n)-B(n)-C(n)-D(n)\cdot d-\left(A(0)-B(0)-C(0)-D(0)\cdot d\right) \\
          &= A(n)-0-X-D(n)\cdot d-\l开发者_如何学编程eft(0-0-0-0\right) \\
          &= A(n)-X-D(n)\cdot d
\end{align*}

The problematic line is the first line, which is too long.

The breqn package is designed to split long equations automatically. It works very well in the majority of situations, but it's not as mature as the amsmath package. Here's how you'd write your example equation:

\documentclass{article}
\usepackage{breqn}
\begin{document}
\begin{dmath}
f(n)-f(0) = A(n)-B(n)-C(n)-D(n)\cdot d-\left(A(0)-B(0)-C(0)-D(0)\cdot d\right)
          = A(n)-0-X-D(n)\cdot d-\left(0-0-0-0\right)
          = A(n)-X-D(n)\cdot d
\end{dmath}
\end{document}

Note there is no markup for alignment or newlines, but the output looks essentially the same as if you used align.

I usually prefer to handle this by using the amsmath package and using the split structure. There are a bunch of useful structures in there for splitting equations across lines, but that's usually the simplest to use.

Many TeX installations will already have the package, but you can also get it from the AMS website.

The standard approach I've used in the past is an eqnarray. See for example this page.

As far as I know, this is not possible. When working inside a display, you are responsible for line breaks. How to line break, and how to continue on the next line in case of brackets, is a tough question for humans (check, for instance, the relevant section in Grätzer, "Math into LaTeX"), let alone for a computer.

Example: when you break the first line after \left(, you need a \right. at the end, and \left. at the beginning of the next line (otherwise you'll get an error). Moreover, you'd want the beginning of the next line to be further right than the bracket produced by \left(