# Splitting a square root over two lines

I was wondering how i would replicate something like this, having a lot of trouble trying to this using multiline

This is where i'm at so far

\begin{multlined}
E_{\pm} = \pm t_{1}
\sqrt{ 3 + 2\cos {\frac {\sqrt{3} k_{x} a }{2}+\frac{k_{y} a}{2} } \\
+2 \cos{ \frac{ \sqrt{3}k_{x}a}{2}-\frac{k_{y}a}{2}}+2\cos{k_{y} a}}.
\end{multlined}


You can use aligned.

A few points:

1. I used \; to further separate the coefficient from the square root (but probably I'd use \frac{E_{\pm}}{\pm t_1} to make things clearer)

2. Inside aligned I used \tfrac to reduce the visual clutter.

3. \cos doesn't take an argument; less braces in the source.

4. You forgot a few parentheses.

\documentclass[a4paper,twocolumn]{article}
\usepackage{newtxtext,newtxmath}
\usepackage{amsmath}

\usepackage{lipsum}

\begin{document}

\lipsum[22][1-3]
\hspace{0pt} E_{\pm} = \pm t_{1} \; \sqrt{ \begin{aligned} 3 & + 2\cos \Bigl(\tfrac {\sqrt{3} k_{x} a }{2}+\tfrac{k_{y} a}{2}\Bigr) \\ & +2 \cos \Bigl(\tfrac{ \sqrt{3}k_{x}a}{2}-\tfrac{k_{y}a}{2}\Bigr) + 2\cos(k_{y}a) \end{aligned} }. \hspace{1000pt minus 1fill}
\lipsum[42][1-3]

\end{document}


The \hspace tricks are because otherwise the equation number would be moved down. Try without them first: the need may arise depending on your column width.

\documentclass{article}

\begin{document}

$E_{\pm} = \pm t_{1} \sqrt{% \begin{array}{l} 3 + 2\cos {\frac {\sqrt{3} k_{x} a }{2}+\frac{k_{y} a}{2} } \\ {} +2 \cos{ \frac{ \sqrt{3}k_{x}a}{2}-\frac{k_{y}a}{2}}+2\cos{k_{y} a} \end{array}% }$

\end{document}