1／3+1／15+1／35+1／63+1／99
=1/3+1/3-1/5+1/5-1/7+1/7+1/9+1/9-1/11
=2/3-1/11=21/331/3-1/5≠1／15......啊？看错了，脑筋短路，还有个1/2