1/3+1/15+1/35+1/63+1/99
=[（1-1/3）+（1/3-1/5）+（1/5-1/7）+（1/7-1/9）+（1/9-1/11）]÷(3-1)
=[1-1/3+1/3-1/5+1/5-1/7+1/7-1/9+1/9-1/11]÷2
=(1-1/11)÷2
=5/11
裂项分拆:
1/(a×b)=(1/a-1/b)÷(b-a)