Toán nh 1/4+1/9+116+…+1/81 Chứng tỏ 8\9 >A>2\5 16/09/2021 By Josephine nh 1/4+1/9+116+…+1/81 Chứng tỏ 8\9 >A>2\5
Đáp án + giải thích bước giải : `A = 1/4 + 1/9 + 1/16 + … + 1/81` `-> 1/(2 . 3) + 1/(3 . 4) + … + 1/(9 . 10) < A < 1/(1 . 2) + 1/(2 . 3) + … + 1/(8 . 9)` Xét `1/(2 . 3) + 1/(3 . 4) + … + 1/(9 . 10) < A` `⇔ 1/2 – 1/3 + 1/3 – 1/4 + … + 1/9 – 1/10 < A` `⇔ 1/2 + (- 1/3 + 1/3 – 1/4 + … + 1/9) – 1/10 < A` `⇔ 1/2 – 1/10 < A` `⇔ 2/5 < A (1)` Xét `A < 1/(1 . 2) + 1/(2 . 3) + … + 1/(8 . 9)` `⇔ A < 1 – 1/2 + 1/2 – 1/3 + …. + 1/8 – 1/9` `⇔ A < 1 + (- 1/2 + 1/2 – 1/3 + …. + 1/8) – 1/9` `⇔ A < 1 – 1/9` `⇔ A < 8/9 (2)` Từ `(1), (2) ⇒ 2/5 < A < 8/9` Trả lời
Ta có: `1/2.2< 1/1.2 ; 1/3.3 < 1/2.3; ….. ; 1/9.9 < 1/8.9` =>` 1/2^2 + 1/3^2 + 1/4^2+………+ 1/9^2 < 1/1.2 + 1/2.3 + …. + 1/8.9` => `A < 1/1 -1/2 + 1/2 -1/3 +….+ 1/8- 1/9` =>`A < 1/1 -1/9` => `A < 8/9` (1) Lại có: `1/2.2 > 1/2.3 + 1/3.3 > 1/3.4 ; ………; 1/9.9 > 1/9.10` => `1/2^2 + 1/3^2 + 1/4^2+………+ 1/9^2 > 1/2.3 + 1/3.4 +…+ 1/9.10` => `A > 1/2 -1/3 + 1/3 -1/4 +….+ 1/9-1/10` => `A > 1/2 -1/10 = 2/5` (2) Từ (1) và (2) => `8/9 > A > 2/5` Vậy `8/9 > A > 2/5` Trả lời
Đáp án + giải thích bước giải :
`A = 1/4 + 1/9 + 1/16 + … + 1/81`
`-> 1/(2 . 3) + 1/(3 . 4) + … + 1/(9 . 10) < A < 1/(1 . 2) + 1/(2 . 3) + … + 1/(8 . 9)`
Xét `1/(2 . 3) + 1/(3 . 4) + … + 1/(9 . 10) < A`
`⇔ 1/2 – 1/3 + 1/3 – 1/4 + … + 1/9 – 1/10 < A`
`⇔ 1/2 + (- 1/3 + 1/3 – 1/4 + … + 1/9) – 1/10 < A`
`⇔ 1/2 – 1/10 < A`
`⇔ 2/5 < A (1)`
Xét `A < 1/(1 . 2) + 1/(2 . 3) + … + 1/(8 . 9)`
`⇔ A < 1 – 1/2 + 1/2 – 1/3 + …. + 1/8 – 1/9`
`⇔ A < 1 + (- 1/2 + 1/2 – 1/3 + …. + 1/8) – 1/9`
`⇔ A < 1 – 1/9`
`⇔ A < 8/9 (2)`
Từ `(1), (2) ⇒ 2/5 < A < 8/9`
Ta có: `1/2.2< 1/1.2 ; 1/3.3 < 1/2.3; ….. ; 1/9.9 < 1/8.9`
=>` 1/2^2 + 1/3^2 + 1/4^2+………+ 1/9^2 < 1/1.2 + 1/2.3 + …. + 1/8.9`
=> `A < 1/1 -1/2 + 1/2 -1/3 +….+ 1/8- 1/9`
=>`A < 1/1 -1/9`
=> `A < 8/9` (1)
Lại có: `1/2.2 > 1/2.3 + 1/3.3 > 1/3.4 ; ………; 1/9.9 > 1/9.10`
=> `1/2^2 + 1/3^2 + 1/4^2+………+ 1/9^2 > 1/2.3 + 1/3.4 +…+ 1/9.10`
=> `A > 1/2 -1/3 + 1/3 -1/4 +….+ 1/9-1/10`
=> `A > 1/2 -1/10 = 2/5` (2)
Từ (1) và (2) => `8/9 > A > 2/5`
Vậy `8/9 > A > 2/5`