1/1*6+1/6*11+1/11*16+……+1/51*56=(1/5)*(5/1*6+5/6*11+5/11*16+……+5/51*56)=(1/5)*(1-1/6+1/6-1/11+1/11-1/16+……+1/51-1/56)=(1/5)*(1-1/56)=11/56