1/1*6+1/6*11+1/11*16+.+1/51*56=1/5*(1-1/6)+1/5*(1/6-1/11)+.+1/5*(1/51-1/56)=1/5*(1-1/6+1/6-1/11+1/11-1/16+.+1/51-1/56)中间的都消去了=1/5*(1-1/56)=1/5*55/56=11/56