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I have an equation contained inside \[...\], which automatically makes a \sum with sub- and superscripts turn big--so that the summation sign looks awkward inside parenthesis. Any idea how to make the parenthesis completely enclose the whole summation?

\documentclass{article}
\begin{document}
We have:
\[ \sum_{i=1}^n i = (\sum_{i=1}^{n-1} i) + n =
\frac{(n-1)(n)}{2} + n = \frac{n(n+1)}{2} \]
\end{document}
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4  
Use \left( for the opening bracket and \right) for the closing one. These will automatically adjust their height to fit the contents. –  qubyte Dec 20 '11 at 6:16
    
Related Question: About big parenthesis larger than Bigg. –  Peter Grill Feb 23 '13 at 5:27

3 Answers 3

up vote 59 down vote accepted

The usual thing to do is replace ( with \left( and ) with \right), which automatically expand to fit the material between them. Note that every \left... requires a \right... (but the type of bracket may be different, i.e. \left(...\right] also works).

I would typeset your equation as

\begin{equation*}
\sum_{i=1}^n i = \left(\sum_{i=1}^{n-1} i\right) + n =
\frac{(n-1)(n)}{2} + n = \frac{n(n+1)}{2}
\end{equation*}

enter image description here

For manual control of sizes (most of the time you won't need these)

( \big( \Big( \bigg( \Bigg(

produce

enter image description here

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1  
The environment I use is from the amsmath package, so yours is actually more general. I just find [...] to look messy. Alternatively there is the displaymath environment. –  qubyte Dec 20 '11 at 6:56
1  
@jamaicanworm: amsmath defines \[...\] to be exactly the same as \begin{equation*}...\end{equation*}. See, literally, the last couple of lines of amsmath.sty. –  Werner Dec 20 '11 at 7:26
3  
@jamaicanworm The shape of the brackets is down to the font I'm afraid. If you don't like the larger curved brackets, try the square ones \left[...\right]. These often look better for larger equations. Another thing to take into account is that you're enclosing something relatively thin in those brackets. I suspect it will only look more natural when there's more between them (try putting the n to the left of the sum and loosing the brackets altogether). –  qubyte Dec 20 '11 at 7:29
5  
I would be a bit careful using \left/right around sums etc. because they often become too large. I tend to recommend users to: Scale the fences (parenteses and such) such that it is clear to the read what they fence in, but not to such an extend that the fences dominate the expression. –  daleif Dec 20 '11 at 12:36
1  
@MarkS.Everitt I disagree with this comment of yours "most of the times you won't need these" about the \big... family of commands. In fact, I personally believe that most of the times one shouldn't use the \left...\right construct; the family of \big... commands produces much better spacing (both vertically and horizontally) in most situations; besides you don't need to match them across lines in a display. –  Gonzalo Medina Apr 12 '13 at 20:19

One way is using \left and \right, followed by the parenthesis you want to use. These are mostly () [] {} \langle\rangle and |. You can also use a . to have no parenthesis displayed, e.g. when you want an opening, but no closing one.

\left( \frac12 \right)
\quad
\left\langle \frac23 \right.
\quad
\left\{ \frac34 \right]

creates

enter image description here

If you want to control the size manually, use (in ascending order) \big, \Big, \bigg, \Bigg.

( \frac12 \big)
\quad
\Bigg\langle \frac23 \big]
\quad
\Big\{ \frac34 \Bigg.

results in

enter image description here

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6  
One should use \bigl in front of the left delimiter and \bigr in front of the right delimiter (similarly for \Bigl-\Bigr and the others). This is important for spacing. –  egreg Dec 20 '11 at 7:40
2  
@egreg: When should one use just plain \big and friends? i.e., without the l|r suffix? –  morbusg Dec 20 '11 at 10:34
3  
\big and friends can go in front of ordinaries: \big/ or \big|. –  egreg Dec 20 '11 at 11:41
3  
@egreg at least as long as \big| is not a part of a pair: \big|-x\big|\neq \bigl|-x\bigr| –  daleif Dec 20 '11 at 12:34

Automatically sized parentheses are obtained with \left and \right, as any LaTeX guide or manual tells.

However, automatic sizing is not good in every case; one of these cases is precisely that of summations with limits above and below: compare the results of

\[
\left( \sum_{i=1}^{n-1} i \right)\biggl(\sum_{i=1}^{n-1} i\biggr)
\]

enter image description here

(the font is that obtained with \usepackage{fouriernc}). In general the second way is to be preferred.

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