Add $a$ squared to $b$ squared to get $c$ squared, i.e.,
  $$ a^2 + b^2 = c^2, $$
  in (Plain) TeX.

  Add $a$ squared to $b$ squared to get $c$ squared, i.e.,
  \begin{displaymath} 
      a^2 + b^2 = c^2,
  \end{displaymath} 
  or \[ \], or
  \begin{equation} 
      a^2 + b^2 = c^2.
  \end{equation}