I have an align like this:

```
\begin{align*}
\sigma &= \sqrt{\sum_x \left[ (x-\mu_x)^2\mathbf{f}(x) \right] } = \sqrt{\frac{1}{10}\sum_x (x-\mu_x)^2 } \\
&= \sqrt{\frac{1}{10}(
(0-.45)^2 + (1-.45)^2 + (2-.45)^2 + (3-.45)^2 + (4-.45)^2 +
(5-.45)^2 + (6-.45)^2 + (7-.45)^2 + (8-.45)^2 + (9-.45)^2 } \\
&= \sqrt{24.6525}
\end{align*}
```

However, this goes off the edge of the page. I wanted to create a multiline inside the middle square root symbol, so I tried this:

```
\begin{align*}
\sigma &= \sqrt{\sum_x \left[ (x-\mu_x)^2\mathbf{f}(x) \right] } = \sqrt{\frac{1}{10}\sum_x (x-\mu_x)^2 } \\
&= \sqrt{\frac{1}{10}(
\begin{split} (0-.45)^2 + (1-.45)^2 + (2-.45)^2 + (3-.45)^2 + (4-.45)^2 + \\
(5-.45)^2 + (6-.45)^2 + (7-.45)^2 + (8-.45)^2 + (9-.45)^2 } \end{split} \\
&= \sqrt{24.6525}
\end{align*}
```

but this gives me errors about mismatched curly braces.

Ideally, I would have something exactly like `\frac`

, except which did not draw the horizontal line between it's operands.

## Best Answer

You can use something like:

This works just like

`align`

but is only usable inside math mode.There's also

`gathered`

which works like`gather`

.