$ =(1-\frac{1}{2})+(1-\frac {1}{6})+(1-\frac {1}{12})+(1-\frac {1}{20})$
$+...+(1-\frac {1}{72})$
$=8-(\frac {1}{2}+\frac {1}{6}+\frac {1}{12}+\frac {1}{20}+...+\frac {1}{72})$
$=8-(1-\frac {1}{2}+\frac {1}{2}-\frac {1}{3}+\frac {1}{3}-\frac {1}{4}+\frac {1}{4}-\frac {1}{5}$
$+...+\frac {1}{8}-\frac {1}{9})$
$=8-\frac {8}{9}$
$=7 \frac {1}{9}$
