因为1/2+1/3+1/6=1.反复用1/n*(n+1)=1/n-1/(n+1)
1/3=1/4+1/12
1/6=1/7+1/42
1/4=1/5+1/20
1/5=1/6+1/30
1/2+1/6+1/30+1/20+1/12+1/7+1/42=1