How to specify height and width of \fbox{}

Question

I am trying to have my text read like this inside a box:

        If A and B are two events that are not mutually exclusive then:

                    $P(A \cup B) = P(A) + P(B) - P(A \cap B)$

Al I have is this, can someone help guide me of hidden commands that I don't know about?

\fbox{If A and B are two events that are not mutually exclusive then $P(A \cup B) = P(A) + P(B) - P(A \cap B)$} 

Answer 1

You can use \fbox and a minipage (or a \parbox) of the desired (fixed) width; another option would be to use the varwidth environment from the varwidth package, so the resulting width is the natural width of the contents. A little example showing both approaches:

\documentclass{article} 
\usepackage{varwidth}

\begin{document} 

\noindent\fbox{\begin{minipage}{\dimexpr\textwidth-2\fboxsep-2\fboxrule\relax}
\centering
If A and B are two events that are not mutually exclusive then: 
\[ 
P(A \cup B) = P(A) + P(B) - P(A \cap B) 
\] 
\end{minipage}}

\begin{center}
\fbox{\begin{varwidth}{\dimexpr\textwidth-2\fboxsep-2\fboxrule\relax}
If A and B are two events that are not mutually exclusive then: 
\[ 
P(A \cup B) = P(A) + P(B) - P(A \cap B) 
\] 
\end{varwidth}}
\end{center}

\end{document}

Using the optional arguments for minipage you can control other attributes of the used box; in particular, the second optional argument allows you to specify the height:

\documentclass{article} 

\begin{document} 

\noindent\fbox{\begin{minipage}[t][3\height][c]{\dimexpr\textwidth-2\fboxsep-2\fboxrule\relax}
\centering
If A and B are two events that are not mutually exclusive then: 
\[ 
P(A \cup B) = P(A) + P(B) - P(A \cap B) 
\] 
\end{minipage}}

\end{document}

Answer 2

There is another posibility with tcolorbox package.

A MWE:

\documentclass{article}
\usepackage[skins]{tcolorbox}
\begin{document}
\begin{tcolorbox}[skin=widget,
boxrule=1mm,
coltitle=black,
colframe=blue!45!white,
colback=blue!15!white,
width=(.9\linewidth),before=\hfill,after=\hfill,
adjusted title={Non Mutually Exclusive Events}]
If A and B are two events that are not mutually exclusive then:  
\tcblower
$P(A \cup B) = P(A) + P(B) - P(A \cap B)$
\end{tcolorbox}
\end{document}

Answer 3

What about framebox ?

\framebox(300,100){some text (or keep it empty)}

where 300 is the wanted width and 100 the height of the box, in \unitlength (pt by default).

(Credits to David Carlisle)

Answer 4

Gonzalo Medina's is the actual answer to the problem given, but here is another option: Instead of using an \fbox you could also the mdframed package which in basic usage

\begin{mdframed}
    If A and B are two events that are not mutually exclusive then: 
    \[ P(A \cup B) = P(A) + P(B) - P(A \cap B) \] 
\end{mdframed}

produces a nice box:

but also allows you to get fancy should you so desire:

Notes:

• The style used here is only slightly modified from one of the examples in the documentation, but am sure you could pick better color choices. This one was just meant to illustrate a few of the options, but there are numerous others.

Code:

\documentclass{article} 
\usepackage{xcolor}
\usepackage[framemethod=tikz]{mdframed}

\begin{document} 
\begin{mdframed}
    If A and B are two events that are not mutually exclusive then: 
    \[ 
    P(A \cup B) = P(A) + P(B) - P(A \cap B) 
    \] 
\end{mdframed}

\bigskip
\begin{mdframed}[
        linecolor=red,linewidth=2pt,% 
        frametitlerule=true,% 
        apptotikzsetting={\tikzset{mdfframetitlebackground/.append style={%
            shade,left color=white, right color=blue!20}}}, 
        frametitlerulecolor=blue,
        frametitlerulewidth=1pt, innertopmargin=\topskip,
        frametitle={Non Mutually Exclusive Events},
        outerlinewidth=1.25pt
    ]
    % ----------
    If A and B are two events that are not mutually exclusive then: 
    \[ 
    P(A \cup B) = P(A) + P(B) - P(A \cap B) 
    \] 
\end{mdframed}
\end{document}