=1/5×6+1/6×7+1/7×8-1/8×9
=1/5-1/6+1/6-1/7+1/7-1/8+1/8-1/9
=1/5-1/9
=4/45
简便计算：1/30+1/42+1/56+1/72=
简便计算：1/30+1/42+1/56+1/72=
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急
数学人气：453 ℃时间：2020-01-14 13:03:07
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