# Multiple lines in equation environment

I have to put an two equations inside one equation environment. I could do it with this piece o code:

$$\begin{array}{ccl} \nabla \cdot F & = & \left(\frac{\partial}{\partial x}i + \frac{\partial}{\partial y}j + \frac{\partial}{\partial z}\right) \cdot (F_{1}i + F_{2}j + F_{3}k) \\ & = & \frac{\partial F_{1}}{\partial x}+ \frac{\partial F_{2}}{\partial y} + \frac{\partial F_{3}}{\partial z} \end{array}!$$


Which gives me this image:

The problem is cause when I use the array, my fractions get smaller. Here is a single line equation and how I would like to preserve my fraction size in a environment with more than 1 equation.

Can I achieve this result?

• If you're using amsmath, then use \dfrac instead of \frac. However, for such alignment, it's easier to use \begin{align}...\end{align}, unless you want to number the entire multi-line equation with a single tag. Then you need something different. – Werner Apr 14 '14 at 18:09
• Yes, and want to number the entire multi-line equation. The \dfrac worked, but it seems that the lines are get over each other. – pceccon Apr 14 '14 at 18:17

You should try \dfrac{}{} instead of \frac{}{}. Include the amsmath (http://www.ctan.org/pkg/amsmath) package.

The "d" stands for displaysize(or style), you can of course do a \displaystyle on every symbol, but that would be too clumsy.

EDIT: Corrected on Steven B. Segletes suggestion.

Here is a reworking of what you want to achieve using amsmath's aligned environment:

\documentclass{article}
\usepackage{amsmath}% http://ctan.org/pkg/amsmath
\newcommand{\pfrac}[2][]{\frac{\partial #1}{\partial #2}}% \pfrac[<top>]{<bottom>}
\begin{document}
\begin{aligned} \nabla \cdot F &= \biggl( \pfrac{x}i + \pfrac{y}j + \pfrac{z} \biggr) \cdot \bigl(F_1 i + F_2 j + F_3 k \bigr) \\ &= \pfrac[F_1]{x}+ \pfrac[F_2]{y} + \pfrac[F_3]{z} \end{aligned}
\end{document}


The addition of \pfrac[<top>}{<bottom>} is to simplify your notation and provide consistency.