I am trying to split my equation. I am using the code below. However, each time I am getting the following error

! Extra }, or forgotten \right. } $\hfill \endtemplate

l.215 \end{split} ?

```
\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{equation}\begin{split}
MSE_{II}(\overline{y}_{CST})&= MSE_{I}(\overline{y}_{CST})+\overline{Y}^{2}\left[\left(c_{2}^{2}+2c_{1}c_{3}\right)V_{4,\, 0}+2c_{1}c_{2}V_{3,\, 0}+2\left(2c_{1}c_{2}+c_{3}\right)V_{3,\, 1} +\\
&+2 \left(c_{1}^{2}+c_{2} \right)V_{2,\, 1}+\left( c_{1}^{2}+2c_{2} \right)V_{2,\, 2}+{2c_{1}V_{1,\, 2}}\right]
\end{split}\end{equation}
\end{document}
```

But, when I remove, `\left[`

and `\right]`

codes, it works fine. I do not understand. Please, help me.

## Best Answer

Besides the balancing issue associated to the use of

`\left`

,`\right`

, your expression can be improved introducing some modifications:`\left( ... \right)`

for the inner expressions produces delimiters that are too big and introduce some additional horizontal space that it's superfluous in this particular case, so I think that it's better to use just`(...)`

.`+{}`

so that the ending plus sign in the first line is correctly trated as an operator; analogously, you need to use`{}+`

for the first plus sign in the second line.`\left[`

,`\right.`

in the first line and`\right.`

,`\left]`

in the second line, I would suggest you to use`\bigl[`

,`\bigr]`

which don't need to be balanced in each line and produce better sized delimiters in most cases.In the following code I include, for comparison, your code (with the balancing problem solved) and then the code implementing my suggestions: