Custom Command cannot use equation mode?

Question

I have defined a new command below, but when ever I pass an equation into, it will not work.

\newcommand{\defn}[1]{
{\textbf{Definition}}\\  
\parbox{5.2in}{\begin{flushright} \fbox{#1}\end{flushright} } }

This is what it allows me to do:

\defn{If $f$ is integrable over the interval $[a,b]$, we define $\int_{b}^{a} f(x)\ dx$ }

However, this is what I want to do:

\defn{If $f$ is integrable over the interval $[a,b]$, we define 
$$\int_{b}^{a} f(x)\ dx$$ }

Or alternatively,

 \defn{If $f$ is integrable over the interval $[a,b]$, we define 
  \begin{equation*}
    \int_{b}^{a} f(x)\ dx
  \end{equation}}

I would greatly appreciate any help. Thank you.

Answer 1

Try:

\documentclass{article}
\newcommand{\defn}[1]{%
\textbf{Definition:}\par  
\minipage{5.2cm}
#1
\endminipage
}

\begin{document}
 \defn{If $f$ is integrable over the interval $[a,b]$, we define 
  \begin{equation}
    \int_{b}^{a} f(x)\ dx
  \end{equation}
}

\end{document}

You might also consider the following alternate definition, where you can set the size as we:

\newcommand{\defn}[2]{%
\textbf{Definition:}\par  
\hsize#1
#2
}