How do I break a long equation over lines?

Question

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-\left(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.

Answer 1

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.