1/20+1/30+1/42+1/56简便计算,

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1/20+1/30+1/42+1/56简便计算,1/20+1/30+1/42+1/56简便计算,1/20+1/30+1/42+1/56简便计算,解原式=(1/4-1/5)+(1/5-1/6)+(1/6

1/20+1/30+1/42+1/56简便计算,
1/20+1/30+1/42+1/56
简便计算,

1/20+1/30+1/42+1/56简便计算,

原式
=(1/4-1/5)+(1/5-1/6)+(1/6-1/7)+(1/7-1/8)
=1/4+(1/5-1/5)+(1/6-1/6)+(1/7-1/7)-1/8
=1/4-18
=1/8

3/4+1/20+1/30+1/42+1/56+1/72
=(1-1/4)+(1/4-1/5)+(1/5-1/6)+(6/1-1/7)+(1/7-1/8)+(1/8-1/9)
=1-1/4+1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/8+1/8-1/9
=1-1/9
=8/9
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