`S=7/(2.5)+7/(5.8)+7/(8.11)+………+7/(65.68)` `3/7S=3/(2.5)+3/(5.8)+3/(8.11)+…+3/(65.68)` `3/7S=1/2-1/5+1/5-1/8+1/8-1/11+…+1/65-1/68` `3/7S=1/2-1/68` `3/7S=33/68` `S=77/68` Bình luận
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`S=7/(2.5)+7/(5.8)+7/(8.11)+………+7/(65.68)`
`3/7S=3/(2.5)+3/(5.8)+3/(8.11)+…+3/(65.68)`
`3/7S=1/2-1/5+1/5-1/8+1/8-1/11+…+1/65-1/68`
`3/7S=1/2-1/68`
`3/7S=33/68`
`S=77/68`