1/4 + 1/8 + 1/16 +1/32 + 1/64 + 1/128 + 1/256 + 1/512

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1. Mình giải theo cách lớp `5` nha!

   Ta thấy mỗi số hạng trong dãy gấp `2` lần phân số sau nó

   Gọi : `A=1/4+1/8+1/16+1/32+1/64+1/128+1/256+1/512`

   `⇒2×A=1/2+1/4+1/8+1/16+1/32+1/64+1/128+1/256`

   `⇒2×A-A=(1/2+1/4+1/8+1/16+1/32+1/64+1/128+1/256)-(1/4+1/8+1/16+1/32+1/64+1/128+1/256+1/512)`

   `⇒A=1/2+1/4+1/8+1/16+1/32+1/64+1/128+1/256-1/4-1/8-1/16-1/32-1/64-1/128-1/256-1/512`

   `⇒A=1/2-1/512`

   `⇒A=256/512-1/512`

   `⇒A=255/512`

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