1/20+1/30+1/42+1/56+1/72=1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/8+1/8-1/9=1/4-1/9=5/36
1/4-1/5+1/5-1/6+...1/8-1/9=1/4-1/9=5/36
=1/4×5+1/5×6+1/6×7+1/7×8+1/8×9=1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/8+1/8-1/9=1/4-1/9=5/36
