Đáp án:
a, `text{ Để A nhận giá trị nguyên thì:}`
`3n + 2 \vdots n`
`3n + 2 - 3n - 6 \vdots n`
`-4 \vdots n`
`⇒ n \in Ư(-4)`
`Ư-(4) = { - 4 ; - 2 ; - 1 ; 1 ; 2 ; 4}`
`⇒ n \in {-4 ; -2 ; -1 ; 1 ; 2 ; 4}`
b, `A = 1 + 1/8 + 1/24 + 1/48 + 1/80 + 1/120`
` = 1 + (1/8 + 1/24 + 1/48 + 1/80 + 1/120)`
` = 1 + (1/(2 . 4) + 1/(4 . 6) + 1/(6 . 8) + 1/(8 . 10) + 1/(10 . 12))`
` = 1 + 1/2 . (2/(2.4) + 2/(4.6) +2/(6.8) + 2/(8.10) + 2/(10.12))`
` = 1 + 1/2 . (1/2 - 1/4 + 1/4 - 1/6 + ....... + 1/10 - 1/12)`
` = 1 + 1/2 . (1/2 - 1/12)`
` = 1 + 1/2 . 5/12`
` = 1 + 5/24 = 29/24`
