=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
20=4*5 1/20=1/4-1/5
30=5*6 1/30=1/5-1/6
42=6*7 1/42=1/6-1/7
56=7*8 1/56=1/7-1/8
72=8*9 1/72=1/8-1/9
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+(-1/5+1/5-1/6+1/6-1/7+1/7-1/8+1/8)-1/9
=1/4-1/9=5/36
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