1/20+1/30+1/42+……+1/9900
=1/(4×5)+1/(5×6)+1/(6×7)+……+1/(99×100)
=(1/4-1/5)+(1/5-1/6)+(1/6-1/7)+……(1/99-1/100)
=1/4-1/5+1/5-1/6+1/6-1/7+……+1/99-1/100
=1/4-1/100
=25/100-1/100
=24/100
=6/25
1/20+1/30+1/42+……+1/9900等于多少?
3个回答
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