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\begin{document}

\[
\sum a^2\frac{x^2}{x^2} \quad 
\textstyle \sum a^2\frac{x^2}{x^2}\quad
\scriptstyle \sum a^2\frac{x^2}{x^2}\quad  
\scriptscriptstyle\sum a^2\frac{x^2}{x^2} 
\]

\end{document}