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\begin{document}
$$
U={\frac  {A_{{t}}}{A_{{t}}+2A_{{p}}+A_{{t_{{ACK}}}}}}={\frac  {A_{{t}}}{A_{{t}}+2A_{{t}}/2+A_{{t_{{ACK}}}}}}={\frac  {A_{{t}}}{2A_{{t}}+A_{{t_{{ACK}}}}}}<{\frac  {1}{2}}
$$
\end{document}
