1/5乘6乘7+1/6乘7乘8+1/7乘8乘9+1/8乘9乘10+1/720
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1/(5×6×7)+1/(6×7×8)+1/(7×8×9)＋1/(7×8×9)＋1/720
=[1/(5×6)-1/(6×7)]/2+[1/(6×7)-1/(7×8)]/2+[1/(7×8-1/(8×9)]/2＋1/720＋1/720
=[1/(5×6)-1/(8×9)]/2＋1/360
=1/15-1/36
＋1/360
=7/180＋1/360
=14/360＋1/360
=1/24
=[1/(5×6)-1/(6×7)]/2+[1/(6×7)-1/(7×8)]/2+[1/(7×8-1/(8×9)]/2＋1/720＋1/720
=[1/(5×6)-1/(8×9)]/2＋1/360
=1/15-1/36
＋1/360
=7/180＋1/360
=14/360＋1/360
=1/24
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