created at 05-18-2021
views: 279

To align equations, we use the **align** environment, and to use align environment, package `amsmath`

should be called.

**this method uses** `\{split}`

and `={}&`

in a `\align`

environment to align equations

```
\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{align}
\begin{split}
equation1 ={}& ...
\end{split} \\
\begin{split}
equation2 ={}& ...
\end{split}\\
equation3 ={}& ...
\end{align}
\end{document}
```

Notes for this method:

**No**blank lines between`\begin{align}`

and`\end{split}`

- the last equation isn't wrapped in
`{split}`

environment. - add
`{}&`

to equal sign of each equation:`={}&`

- line break symbols
`\\`

after each equation or`\end{split}`

- these tips must be noted, or you will get errors or disordered layout.

```
\documentclass[12pt]{article}
\usepackage{amsmath}
\begin{document}
\begin{align}
\begin{split}
g(x) ={}& 2[1-(1+x)exp(-x)]/x^2
\end{split}\\
g'(x) ={}& -2[1-(1+x+x^2/2)exp(-x)]/x^2
\end{align}
\end{document}
```

Different from **method #1**, replace `={}&`

with `&=`

, and no need for `split`

environment:

```
\documentclass[12pt]{article}
\usepackage{amsmath}
\begin{document}
\begin{align}
g(x) &= 2[1-(1+x)exp(-x)]/x^2\\
g'(x) &= -2[1-(1+x+x^2/2)exp(-x)]/x^2
\end{align}
\end{document}
```

also note:

**No**blank lines from`\begin{split}`

to`\end{align}`

- use
`\\`

to make line breaks - add
`&`

to each equal sign of equations

Actually, in above examples, we have at least two conditions to be considered:

- align
**with**number, or - align
**without**number

If you don't want equations to be numbered, you just need add a `*`

symbol to the `{align}`

environment, that is:

```
\begin{align*}
[content...]
\end{align*}
```

this works for both of the methods mentioned above. The rest elements between `\begin{align*}` and `\end{align*}`are all the same as above numbered alignment.

---

Long equations in latex will arise several problems, so here we treat long equations specially.

For long equations, we have to use `\\`

to make new lines so that the whole equation can be displayed properly. However, this action also cause our single equation be numbered duplicately:

to solve this problem, we use a `\nonumber`

mark at the end of lines and before the line break mark `\\`

, except for the one we want the number be displayed at (often the last line):

```
\begin{align}
[line1...] \nonumber \\
[line2...] \nonumber \\
[line3...] % show number at this line
\end{align}
```

now, duplicate numbers disappear.

Next, we will adjust the alignment to make the display of the number more proper.

Alignment of equations in former sections are based on the equal sign `=`

, different equations can be aligned along the vertical line of "=". However, for long equations, how do we adjust multiple lines of a single equation when there is only one equal sign?

Based on last example, we use two key marks to align a long equation with line breaks:

`&=`

`\mathrel{phantom{=}}`

`\phantom{}`

: is used to make space,`phantom{=}`

means make a space of which the width is equal to a`=`

sign.`\mathrel{}`

: make relation spacing.

syntax:

```
\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{align}
[left] &= [line1...] \nonumber \\
&\mathrel{\phantom{=}} [line2...] \nonumber \\
&\mathrel{\phantom{=}} [line3...] % show number at this line
\end{align}
\end{document}
```

now, the example equation has a good appearance:

- note the
`&`

sign before`\mathrel`

In some cases we need to use equal sign multiple times in a single equation, how do we align the equation then?

Since there are equal sign in each line, so the alignment would be easily base on the equal sign again:

syntax:

```
\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{align}
[left] &= [line1...] \nonumber \\
&= [line2...] \nonumber \\
&= [line3...] % show number at this line
\end{align}
\end{document}
```

(note: the example is a fake equation).