因为1/30=1/5-1/6;1/42=1/6-1/7;1/56=1/7-1/8;1/72=1/8-1/9
所以,原式=1/5-1/6+1/6-1/7+1/7-1/8+1/8-1/9
=1/5-1/9
=4/45