Align equations over multiple sections

Question

Is there a way to make the math mode "align" work over multiple sections? Here is the code example:

section*{Polynomials}
Do the following polynomial devisions
\begin{align}
&(x^7 - x^4 - 21x^2 + 4x + 18) \div (x^2-9) \tag{a} \\
&7x^3 - 6x^2 - 2x + 8 \div (x-1) \tag{b}
\end{align}
\section*{Simplifying}    
\begin{align}
Rewrite the follwing equations as sums
& (g^7+c)\cdot (f+\pi k^{-7}+3jg)-\pi-8hb^2-er^2 \tag{a}\\
& \beta^{56}\cdot (x^5-\frac{85l^8}{\beta^9}+\beta^{-54}) \tag{b}
\end{align}

The problem now is that the equations are only aligned in each section (as they should be, so that works).

Meaning the equations in section "Polynomials" so not align (or start at the same place) as the ones in the next section "Simplifying".

Is there a way to do this?

Answer 1

Your objective may be achieved easily by placing the sectioning instructions and short paragraphs inside an \intertext wrapper. In the example below, there is now only one align environment that spans a \section* directive. Do note that some material can not be placed inside \intertext.

To play it safe, one should probably provide the instruction \allowdisplaybreaks to allow page breaks within what may turn out to be a rather lengthy align environment.

Two asides:

• The example below shows a single sectioning header included in the large align environment. There is no LaTeX-based reason for not having a single align environment span multiple pages and includes several sectioning headers. Of course, there will most likely be reasons related to document content and layout that make it unnecessary -- or undesirable -- to have an align environment that stretches across a multitude of pages.

• Just because sectioning headers can be placed in the argument of \intertext does not imply that everything does. For instance, as @eldering has noted in a comment, \footnote commands should not be placed in the argument of \intertext (unless one resorts to some pretty fancy workarounds, I suppose).

\documentclass{article}
\usepackage{amsmath}
\allowdisplaybreaks
\begin{document}
\section*{Polynomials}
Do the following polynomial devisions
\begin{align}
&(x^7 - x^4 - 21x^2 + 4x + 18) \div (x^2-9) \tag{a} \\
&7x^3 - 6x^2 - 2x + 8 \div (x-1) \tag{b}
\intertext{%
\section*{Simplifying}
Rewrite the follwing equations as sums}
& (g^7+c)\cdot (f+\pi k^{-7}+3jg)-\pi-8hb^2-er^2 \tag{a}\\
& \beta^{56}\cdot (x^5-\frac{85l^8}{\beta^9}+\beta^{-54}) \tag{b}
\end{align}
\end{document}

Answer 2

The Answer of Mico will definitly work, but will be very costly if you'll write a much longer piece of text (e.g. Master Thesis). Using a koma-script class with the option fleqn (flush left equation) will "align" all equations on the left side:

\documentclass[fleqn]{scrartcl}
\usepackage{amsmath}

\begin{document}
\section*{Polynomials}
Do the following polynomial devisions
\begin{align}
&(x^7 - x^4 - 21x^2 + 4x + 18) \div (x^2-9) \tag{a} \\
&7x^3 - 6x^2 - 2x + 8 \div (x-1) \tag{b}
\end{align}

\section*{Simplifying}
Rewrite the follwing equations as sums
\begin{align}
& (g^7+c)\cdot (f+\pi k^{-7}+3jg)-\pi-8hb^2-er^2 \tag{a}\\
& \beta^{56}\cdot (x^5-\frac{85l^8}{\beta^9}+\beta^{-54}) \tag{b}
\end{align}
\end{document}