1/(5*8)=(1/3)*(1/5-1/8)
1/(8*11)=(1/3)*(1/8-1/11)
其他项同理
原式=(1/3)*(1/5-1/8+1/8-1/11+...+1/17-1/20)
=(1/3)*(1/5-1/20)
=1/20