1/30+1/42+1/56+1/72简便算法是?

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1/30+1/42+1/56+1/72简便算法是?1/30+1/42+1/56+1/72简便算法是?1/30+1/42+1/56+1/72简便算法是?1/30+1/42+1/56+1/72=1/5*6

1/30+1/42+1/56+1/72简便算法是?
1/30+1/42+1/56+1/72简便算法是?

1/30+1/42+1/56+1/72简便算法是?
1/30+1/42+1/56+1/72
=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
=9/45-5/45
=4/45

1/30+1/42+1/56+1/72
=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