一道简算题!1/30+1/45+1/56+1/72一定要简算!

一道简算题!1/30+1/45+1/56+1/72一定要简算!
一道简算题!1/30+1/45+1/56+1/72
一定要简算!

一道简算题!1/30+1/45+1/56+1/72一定要简算!
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

第二个数应该是1／42吧？