# Parentheses around mismatched size fractions in LaTeX

1. ### wolfbd

1
I have a fraction in the denominator of another fraction, and I'm trying to put a set of brackets around it. However, I can't seem to get them to size properly. Example below:

Code (Text):
Q_1 \left[ \frac{Q_2}{4\pi \left( r_2+\sqrt{ \dfrac{Q_2\gamma A}{4\pi}} \right)^2 } + Q_3 \right]
which comes out as

Q_1 \left[ \frac{Q_2}{4\pi \left( r_2+\sqrt{ \dfrac{Q_2\gamma A}{4\pi}} \right)^2 } +Q_3\right]

Obviously, I want to get rid of the space at the top. I've tried using \Bigg[ (which ends up too small) and even creating my own sizing in the preamble:
Code (Text):
\makeatletter
\newcommand{\vast}{\bBigg@{4}}
\makeatother

(which ends up too big since it only accepts integer sizing, as far as I can tell). Any ideas? Thanks.

2. ### AlephZero

7,298
You can get the brackets right by putting the fraction inside a matrix.

That leaves the ##Q_1## in a silly place, but you can fix that with the \vphantom{} command. \vphantom{} works out the vertical height of what is inside the {}, and creates an invisible zero-width object of that size.

So, in front of the matrix in [ ] , make another matrix without backets, use \vphantom to make it the same height, and the ##Q_1## will line up with the ##Q_3##.

Code (Text):
\begin{matrix}
\vphantom{\frac{Q_2}{4\pi \left( r_2+\sqrt{ \dfrac{Q_2\gamma A}{4\pi}} \right)^2 }}
Q_1
\end{matrix}
\begin{bmatrix}
\frac{Q_2}{4\pi \left( r_2+\sqrt{ \dfrac{Q_2\gamma A}{4\pi}} \right)^2 } + Q_3
\end{bmatrix}

$$\begin{matrix} \vphantom{\frac{Q_2}{4\pi \left( r_2+\sqrt{ \dfrac{Q_2\gamma A}{4\pi}} \right)^2 }} Q_1 \end{matrix} \begin{bmatrix} \frac{Q_2}{4\pi \left( r_2+\sqrt{ \dfrac{Q_2\gamma A}{4\pi}} \right)^2 } + Q_3 \end{bmatrix}$$

if you are a perfectionist, you might want to put a bit of negative space in between the two matrices as well.

Easy peasy.

Last edited: May 2, 2012

### Staff: Mentor

One problem is that you are fighting LaTeX by using \dfrac. Simply changing to \frac improves things to some extent:

$$Q_1 \left[ \frac{Q_2}{4\pi \left( r_2+\sqrt{ \frac{Q_2\gamma A}{4\pi}} \right)^2 } + Q_3 \right]$$

There are other ways to represent division. Sometimes $a/b$ looks better than $\frac a b$:
$$Q_1 \left[ \frac{Q_2}{4\pi \left( r_2+\sqrt{ (Q_2\gamma A)/(4\pi)} \right)^2 } + Q_3 \right]$$

You can pull the $4\pi$ inside the parentheses as $\sqrt{4\pi}$. This clears the denominator that is the root cause of your problems:
$$Q_1 \left[ \frac{Q_2}{\left( r_2\sqrt{4\pi}+\sqrt{Q_2\gamma A} \right)^2 } + Q_3 \right]$$

Sometimes \left and \right are too big. This is one of those times. Use \bigl and \bigr instead:
$$Q_1 \left[ \frac{Q_2}{\bigl( r_2\sqrt{4\pi}+\sqrt{Q_2\gamma A} \bigr)^2 } + Q_3 \right]$$