随便拆一个
利用1/a=1/(a+1)+1/a(a+1)
1=1/2+1/2
=1/2+1/3+1/6
=1/2+1/4+1/12+1/7+1/42
=1/3+1/6+1/5+1/20+1/13+1/156+1/8+1/56+1/43+1/1806