(1+1/2+1/3+1/4)x(1/2+1/3+1/4+1/5)-(1+1/2+1/3+1/4+1/5)x(1/2+1/3+1/4)=1*(1/2+1/3+1/4+1/5)+(1/2+1/3+1/4)*(1/2+1/3+1/4+1/5)-1*(1/2+1/3+1/4)-(1/2+1/3+1/4+1/5)-(1/2+1/3+1/4)=1*(1/2+1/3+1/4+1/5-1/2-1/3-1/4)+...