$由(1)得，\frac{1} {105} = \frac{2} {5 ×6 ×7} = \frac{1} {5 ×6} - \frac{1} {6 ×7}= {\frac{1} {30}}-{\frac{1} {42}},\ $
${\frac{1} {168}} = {\frac{2} {6 ×7 ×8}} = {\frac{1} {6 ×7}}-{\frac{1} {7 ×8}} = {\frac{1} {42}}- {\frac{1} {12}}\frac{1} {56},$
$所以原式=\frac{1} {2}-\frac{1} {6}+\frac{1} {6}-\frac{1} {12}+\frac{1} {12}-\frac{1} {20}+\frac{1} {20}-\frac{1} {30}+\frac{1} {30}-\frac{1} {42}+\frac{1} {42}-\frac{1} {56}\ $
$= \frac{1} {2}-\frac{1} {56}$
$\ = \frac{27} {56}。$
