How to display right brace correctly?

Question

I've got this document class and these math libraries:

\documentclass[12pt,twoside,a4paper]{book}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{cancel}

I'm trying to make something like this:

This is the code I am using right now:

\[ \left.
    \begin{aligned}
        f(x) &= x - 1; \qquad x \in \left< 1;6 \right> \\
        g(x) &= \frac{x^2 - 1}{x + 1}; \quad ~ x \in \left< 1;6 \right>
    \end{aligned}
\qquad \right} f = g? \]

And this is the output:

I couldn't find an answer to what I would like to achieve. The right brace doesn't show up and I worked around the alignment of the function input definition so it appears right below it. I tried \begin{array}{ll}, but then it doesn't align along the equality sign and it makes the fraction smaller, which is not what I want (and the right brace still doesn't work). I don't actually need to find a better way to align the function input definition next to equality sign aligned equations of functions (although it would be nice), but that non-functioning right brace does indeed bug me.

Answer 1

I'd like to recommend that you employ a drcases environment, which is provided by the mathtools package.

Oh, and do please write \langle and \rangle instead of \left< and \right>. Even though using \left< and \right> to create angle brackets kind of "works" for sighted readers, i.e., for people whose eye sight is ok, it's discouraged nowadays. Why? If there's any chance at all that your document will one day be passed through software to make it accessible to unsighted people via voice screen reader software, the software will likely become utterly confused as to how to read \left< and \right> out loud. Please do not think that this possibility is remote. There are currently projects underway to make vast depositories of papers written in PlainTeX and LaTeX, some written years and even decades ago, accessible to unsighted readers via suitable screen reader software. Thus, do try to be thoughtful.

\documentclass[12pt,twoside,a4paper]{book}
\usepackage{mathtools} % for 'drcases' env.
\begin{document}
[
\begin{drcases}
f(x)=x-1 ,;               & x\in\langle1;6\rangle \
g(x)=\frac{x^2-1}{x+1} ,; & x\in\langle1;6\rangle
\end{drcases}
f=g?
]
\end{document}

---

If you truly prefer to pursue your approach, I'd suggest you employ an alignedat{2} environment instead of an aligned environment. No screenshot is presented as the output of the following code block is virtually identical to the screenshot shown above.

\[ 
\left.\begin{alignedat}{2}
        f(x) &= x - 1;                 &     & x \in \langle 1;6 \rangle \\
        g(x) &= \frac{x^2 - 1}{x + 1}; &\quad& x \in \langle 1;6 \rangle
      \end{alignedat}
\right\} 
f = g? 
\]

Answer 2

A solution with drcases from mathtools:

\documentclass[12pt,twoside,a4paper]{book}
\usepackage{mathtools}
\usepackage{amssymb}
\usepackage{cancel}
\begin{document}
[ \begin{drcases}
        f(x) = x - 1; & x \in \langle 1;6 \rangle \quad\
        g(x) = \frac{x^2 - 1}{x + 1}; & x \in \langle 1;6 \rangle
    \end{drcases} \qquad f = g,? ]
\end{document} 

Edit: loading empheq (which loads mathtools) we get almost the same result with the empheq environment:

   \begin{empheq}[right={\empheqrbrace \qquad f = g\,?}]{alignat* = 2}
            & f(x) = x - 1; &\quad & x ∈ \langle 1;6 \rangle \quad\\
             & g(x) = \frac{x² - 1}{x + 1}; & & x ∈ \langle 1;6 \rangle
   \end{empheq}

Answer 3

So thanks for comments, it was really that simple of fixing a simple mistake in code, which is to use \} instead of }:

\[ \left.
    \begin{aligned}
        f(x) &= x - 1; \qquad x \in \left< 1;6 \right> \\
        g(x) &= \frac{x^2 - 1}{x + 1}; \quad ~ x \in \left< 1;6 \right>
    \end{aligned}
\qquad \right\} f = g? \]

Now the output is exactly what I expected: