1/42=1/6*7=1/6-1/7
1/56=1/7*8=1/7-1/8
1/72=1/8*9=1/8-1/9
所以1-1/6+1/42+1/56+1/72=1-1/6+1/6-1/7+1/7-1/8+1/8-1/9=1-1/9=8/9