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`
