1又1/2-5/6+7/12-9/20+11/30-13/42=(1+1/2)-(1/2+1/3)+(1/3+1/4)-(1/4+1/5)+(1/5+1/6)-(1/6+1/7)=1+1/2-1/2-1/3+1/3+1/4-1/4-1/5+1/5+1/6-1/6-1/7=1-1/7=6/7
=630/420-350/420+245/420-189/420+154/420-130/420=6/7
