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)...