1/3+1/15+1/35+1/63+1/99=1/3+1/2×[(1/3-1/5)+(1/5-1/7)+(1/7-1/9)+(1/9-1/11)]=1/3+1/2×(1/3-1/11)=1/3+1/6-1/22=1/2-1/22=10/22=5/11
