# [Tex/LaTex] Breaking equation – Problem with brackets

equations

I want to break a long equation in LaTex. I have tried multline and align but I always get errors with the brackets.

$$\dfrac{dr_{ij}}{dt} = \frac{1}{2r_{ij}} \left[ 2 \left( x_{i}-x_{j} \right) \left( \dfrac{dx_{i}}{dt} - \dfrac{dx_{j}}{dt} \right) + 2 \left( y_{i}-y_{j} \right) \left( \dfrac{dy_{i}}{dt} - \dfrac{dy_{j}}{dt} \right) + 2 \left( z_{i}-z_{j} \right) \left( \dfrac{dz_{i}}{dt} - \dfrac{dz_{j}}{dt} \right) \right]$$


I want to break it in a way, so that all the three "2"s are below each other. How can I do that without getting an error with the brackets?

This is a job for split:
\documentclass{article}
$$\begin{split} \frac{dr_{ij}}{dt} = \frac{1}{2r_{ij}} \biggl[ & 2 \left( x_{i}-x_{j} \right) \left( \dfrac{dx_{i}}{dt} - \dfrac{dx_{j}}{dt} \right) + {}\\ & 2 \left( y_{i}-y_{j} \right) \left( \dfrac{dy_{i}}{dt} - \dfrac{dy_{j}}{dt} \right) + {}\\ & 2 \left( z_{i}-z_{j} \right) \left( \dfrac{dz_{i}}{dt} - \dfrac{dz_{j}}{dt} \right) \biggr] \end{split}$$

Note that we have to manually specify the size for the outermost braces, because \left and \right cannot be divided across lines.