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2乘4分之一加4乘6分之一加6乘8分之一……98乘100分之一 =(1/2)×(1/2-1/4+1/4-1/6+1/6-1/8+…+1/98-1/100) =1/2×(1/2-1/100) =1/2×49/100 =49/200 即200分之49
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