原题应该是：4\3-7\12+9\20-11\30+13\424\3-7\12+9\20-11\30+12\42=(1+1/3)-(1/3+1/4)+(1/4+1/5)-(1/5+1/6)+（1/6+1/7）=1+1/3-1/3-1/4+1/4+1/5-1/5-1/6+1/6+1/7=1+1/7=1又1/7
