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6分之一+12分之一+20分之一+30分之一+.+90分之一 =1/(2×3)+1/(3×4)+1/(4×5)+1/(5×6)+…………+1/(9×10) =1/2-1/3+1/3-1/4+1/4-1/5+…………+1/9-1/10 =1/2-1/10 =8/20 =2/5
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