LaTeX undefined control sequence

latex

I'm working on some homework, and I was checking to make sure my code would compile to a PDF. I continually get an error that reads :

! Undefined control sequence.
<recently read> \urcorner
l.74 (($\urcorner
$
The control sequence at the end of the top line
of your error message was never \def'ed. If you have
misspelled it (e.g., `\hobx'), type `I' and the correct
spelling (e.g., `I\hbox'). Otherwise just continue,
and I'll forget about whatever was undefined.

Here is my code:

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Here is my preamble:

enter image description here

Best Answer

\urcorner is defined by amssymb, which you didn't include in your preamble. So, add

\usepackage{amssymb}

to your preamble and you'll be able to use \urcorner (and \ulcorner, \llcorner and \lrcorner).

Further to your document construction. I'd use an enumerate environment to set your questions. Perhaps also use enumitem to format the labels to suit your needs:

\usepackage{amssymb,enumitem}
...
\begin{enumerate}[label={\arabic*)}]
  \item <some item>
  \item <some other item>
  ...
  \item <last item>
\end{enumerate}

Never just use \\ to end a line.

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