数学 计算 急1/12+1/20+1/30+1/42+1/56+1/72+1/90最好用公分母

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数学 计算 急1/12+1/20+1/30+1/42+1/56+1/72+1/90最好用公分母
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数学 计算 急1/12+1/20+1/30+1/42+1/56+1/72+1/90最好用公分母
数学 计算 急
1/12+1/20+1/30+1/42+1/56+1/72+1/90
最好用公分母

数学 计算 急1/12+1/20+1/30+1/42+1/56+1/72+1/90最好用公分母
1/12+1/20+1/30+1/42+1/56+1/72+1/90
=1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/8+1/8-1/9+1/9-1/10
=1/3-1/10
=7/30

原式=1/(3*4)+1/(4*5)+1/(5*6)+1/(6*7)+1/(7*8)+1/(8*9)+1/(9*10)
=1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/8+1/8-1/9+1/9-1/10
=1/3-1/10
=10/30-3/30
=7/30