1-5/6+7/12-9/20+15/56-12/72
=1-5/6+7/12-9/20+15/56-1/6
=7/12-9/20+15/56
=1/3+1/4-1/4-1/5+15/56
=2/15+15/56
=112/840+225/840
=337/840
1 - 5/6 + 7/12 - 9/20 + 11/30 - 13/42 + 15/56 - 17/72
= 1 - (3+2)/6 + (4+3)/12 - (5+4)/20 + (6+5)/30 - (7+6)/42 + (8+7)/56 - (9+8)/72
= 1 - 1/2 - 1/3 + 1/3 + 1/4 - 1/4 - 1/5 + 1/5 + 1/6 - 1/6 - 1/7 + 1/7 + 1/8 - 1/8 - 1/9
= 1/2 - 1/9
= 9/18 - 2/18
= 7/18
等于十八分之七
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