Scale an equation to fit exact page width

Question

I have an equation that is only a tiny bit too wide for one line. I'd really like to avoid having the equation number on the next line. How do you achieve this? Currently I'm using \small, but this is overkill.

Please note. I have tried scalebox and fittowidth but get errors about missing \endgroup. I have also used \! to its full extent. I am hoping for a solution that will allow me to scale the proper one-line equation to the width of the page.

Here is an example (not my actual equation):

Answer 1

\begin{equation} \resizebox{.9\hsize}{!}{$A+B+C+D+E+F+G+H+I+J+K+L+M+N+O+P+Q+R+S+T+U+V+W+X+Y+Z$} \end{equation} 

or

\begin{equation} \resizebox{.8\hsize}{!}{$A+B+C+D+E+F+G+H+I+J+K+L+M+N+O+P+Q+R+S+T+U+V+W+X+Y+Z$} \end{equation}