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題目上有點問題,那個/1應該是沒有的 原式=1/(1*3)+1/(3*5)+……+1/(9*11) =2*[1/(1*3)+1/(3*5)+……+1/(9*11)]/2 =[2/(1*3)+2/(3*5)+……+2/(9*11)]/2 =(1-1/3+1/3-1/5+1/5-1/7……+1/9-1/11)/2 =(1-1/11)/2 =5/11
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