我怀疑这题目录入有错误,应该是1/30+1/42+1/56+1/72吧?如果是求：1/30+1/42+1/56+1/72计算如下：1/30+1/42+1/56+1/72=1/(5*6)+1/(6*7)+1/(7*8)+1/(8*9)=1/5-1/6+1/6-1/7+1/7-1/8+1/8-1/9=1/5-1/9=4/45...