Parentheses around mismatched size fractions in LaTeX

  1. Apr 29, 2012 #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):
    (which ends up too big since it only accepts integer sizing, as far as I can tell). Any ideas? Thanks.
  2. jcsd
  3. May 2, 2012 #2


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    Science Advisor
    Homework Helper

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

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

    Easy peasy. :devil:
    Last edited: May 2, 2012
  4. May 2, 2012 #3

    D H

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    Staff: Mentor

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

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

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

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

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