\(\frac 1 5.7 \frac 1 7.9+\frac 1 9.11+...+\frac 1 {13}\)
\(\Leftrightarrow\frac 1 2(\frac 2 5.7+ \frac 2 7.9+\frac 2 9.11+...+\frac 1 {13}.15)\)
\(\Leftrightarrow\frac 1 2(\frac 1 5+ \frac 1 7-\frac 1 7+\frac 1 9-\frac 1 9+\frac 1 {11}-\frac 1 {11}+\frac 1 {13}-\frac 1 {13}+\frac 1 {15}\)
\(\Leftrightarrow\frac 1 2(\frac 1 5-\frac1 {15})\)
\(\Leftrightarrow\frac 1 2.\frac2 {15}\)
\(\Leftrightarrow\frac1 {15}\)
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