1/5*1/9+1/9*1/13+1/13*1/17+.1/101*1/105

1/5*1/9=1/4(1/5-1/9);
同理：
1/9*1/13=1/4)(1/9-1/13);
...
1/101*1/105=1/4(1/101-1/105);
1/5*1/9+1/9*1/13+1/13*1/17+.1/101*1/105
=1/4(1/5-1/9+1/9-1/13+...+1/101-1/105);
=1/4(1/5-1/105)=
=5/105
=1/21