"!Missing $ inserted" error using amsmath environments

Question

I am trying to compile a document with equations but whenever I use \begin{eqnarray}..\end{eqnarray} or \begin{align}...\end{align} or anything like this I get the error msg !Missing $ inserted.<inserted text>$. However if I type those equations without using \begin{eqnarray}...\end{eqnarray} or similar things I do not get any error msg and my document runs perfectly.

Even this simple thing does't work....Please help...

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

\begin{document}

\begin{align}
$=$

\end{align}
\end{document}

Answer 1

Many of amsmath environments are already in math mode.

This applies to the environments

• equation,
• align,
• gather,
• flalgin,
• alignat and
• multline

as well as their starred (unnumbered) versions.

The environment split is to be used inside amsmath environments and can be used to split one equation on more than one line (this important for the equation number).

The environments

• aligned,
• gathered and
• alignedat

are similar to the version without -ed but are to be used in math mode.

As pointed out in the comments by Torbjørn T. empty lines aren’t allowed in math mode, they trigger the same error message.

---

amsmath’s environments (those who actually enter math mode) center their contents horizontally. The align family aligns its content internally at every odd &. See the example below how they work. You can use the fleqn option so that amsmath align the whole content of its environment at the left border (with a certain margin).

Code

\documentclass{article}
\usepackage{amsmath}
\usepackage{showframe}
\begin{document}
\begin{equation}
c^2 = a^2 + b^2
\end{equation}

\begin{gather}
            c^2 = a^2 + b^2 \\
a^2 + 2ab + b^2 = (a + b)^2
\end{gather}

\begin{align}
\text{right-aligned} & \neq \text{left-aligned} \nonumber\\
                 c^2 & = a^2 + b^2 \\
     a^2 + 2ab + b^2 & = (a + b)^2
\end{align}

\begin{align}
            c^2 & = a^2 + b^2 \\
\begin{split}
a^2 + 2ab + b^2 & = (a + b)^2 \\
                & = (a + b)(a + b) \\
                & = aa + ab + ba + bb \\
                & = a^2 + 2ab + b^2
\end{split}
\end{align}
\end{document}

Output