1/（6*11）+1/（11*16）+1/（16*21）+1/（21*26）+1/（26*31）+1/（31*36）=1/5*(1/6-1/11)+1/5*(1/11-1/16)+1/5*(1/16-1/21)+1/5*(1/21-1/26)+1/5*(1/26-1/31)+1/5*(1/31-1/36)=1/5*(1/6-1/11+1/11-1/16+1/16-1/21+...