So sánh A=\(\dfrac{3^{2022}+2}{3^{2022}-1}\) với B=\(\dfrac{3^{2022}}{3^{2022}-3}\)
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![](https://rs.olm.vn/images/avt/0.png?1311)
![](https://rs.olm.vn/images/avt/0.png?1311)
Các P/S đó > 3 nhé#
Kí hiệu # : nhận biết đây là tips, câu hỏi, câu trl của riêng mình, tuyệt đối ko copy dưới mọi hình thức. Trừ khi các bn đc sự cho phép của mik^^
>3 nhé
#Ko dựa trên căn bản kĩ thuật nào nên có thể có sai sót mong bn bỏ qua
![](https://rs.olm.vn/images/avt/0.png?1311)
a) Ta có:
2A=2.(12+122+123+...+122020+122021)2�=2.12+122+123+...+122 020+122 021
2A=1+12+122+123+...+122019+1220202�=1+12+122+123+...+122 019+122 020
Suy ra: 2A−A=(1+12+122+123+...+122019+122020)2�−�=1+12+122+123+...+122 019+122 020
−(12+122+123+...+122020+122021)−12+122+123+...+122 020+122 021
Do đó A=1−122021<1�=1−122021<1.
Lại có B=13+14+15+1360=20+15+12+1360=6060=1�=13+14+15+1360=20+15+12+1360=6060=1.
Vậy A < B.
![](https://rs.olm.vn/images/avt/0.png?1311)
3a-b=1/2(a+b)
=>6a-2b=a+b
=>5a=3b
=>a/3=b/5=k
=>a=3k; b=5k
\(A=\dfrac{a^{2022}+3^{2022}}{b^{2022}+5^{2022}}\)
\(=\dfrac{3^{2022}\left(k^{2022}+1\right)}{5^{2022}\left(k^{2022}+1\right)}=\left(\dfrac{3}{5}\right)^{2022}\)
So sánh
A = \(\dfrac{2022^{2023}+1}{2022^{2024}+1}\) và B = \(\dfrac{2022^{2022}+1}{2022^{2023}+1}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
Trước hết ta phải chứng minh \(\dfrac{a}{b}< \dfrac{a+1}{b+1}\) (a, b ϵ N; a < b).
Thật vậy, \(\dfrac{a}{b}=\dfrac{a\left(b+1\right)}{b\left(b+1\right)}=\dfrac{a+ab}{b^2+b}\) và \(\dfrac{a+1}{b+1}=\dfrac{\left(a+1\right)b}{\left(b+1\right)b}=\dfrac{ab+b}{b^2+b}\).
Mà theo giả thuyết là a < b nên \(\dfrac{a+ab}{b^2+b}< \dfrac{ab+b}{b^2+b}\), suy ra \(\dfrac{a}{b}< \dfrac{a+1}{b+1}\) (a, b ϵ N; a < b).
Từ đây ta có:
\(B=\dfrac{2022^{2022}+1}{2022^{2023}+1}=\dfrac{2022^{2023}+2022}{2022^{2024}+2022}=\dfrac{2022^{2023}+2021+1}{2022^{2024}+2021+1}\)
Đặt \(A_1=\dfrac{2022^{2023}+2}{2022^{2024}+2}=\dfrac{2022^{2023}+1+1}{2022^{2024}+1+1}\), rõ ràng \(A_1>A\).
Đặt \(A_2=\dfrac{2022^{2023}+3}{2022^{2024}+3}=\dfrac{2022^{2023}+2+1}{2022^{2024}+2+1}\), rõ ràng \(A_2>A_1\).
...
Đặt \(A_{2020}=\dfrac{2022^{2023}+2021}{2022^{2024}+2021}=\dfrac{2022^{2023}+2020+1}{2022^{2024}+2020+1}\), rõ ràng \(A_{2020}>A_{2019}\) và \(B>A_{2020}\).
Suy ra \(B>A_{2020}>A_{2019}>...>A_2>A_1>A\). Vậy A < B.
Ta có A = \(\dfrac{2022^{2023}}{2022^{2024}}=\dfrac{1}{2022}\) ; B = \(\dfrac{2022^{2022}}{2022^{2023}}=\dfrac{1}{2022}\)
Mà \(\dfrac{1}{2022}=\dfrac{1}{2022}\)
Vậy A = B
![](https://rs.olm.vn/images/avt/0.png?1311)
Lời giải:
$A=\frac{1}{3}+\frac{1}{3^2}+\frac{1}{3^3}+...+\frac{1}{3^{2022}}$
$3A=1+\frac{1}{3}+\frac{1}{3^2}+...+\frac{1}{3^{2021}}$
$\Rightarrow 3A-A=1-\frac{1}{3^{2022}}$
$\Rightarrow A=\frac{1}{2}-\frac{1}{2.3^{2022}}$
Xét hiệu:
$A-B=\frac{1}{2}-\frac{1}{2.3^{2022}}-(1-\frac{1}{3^{2021}})$
$=\frac{1}{3^{2021}}-\frac{1}{2.3^{2022}}-\frac{1}{2}$
$=\frac{5}{2.3^{2022}}-\frac{1}{2}$
$< \frac{1}{2}-\frac{1}{2}=0$
$\Rightarrow A< B$
![](https://rs.olm.vn/images/avt/0.png?1311)
![](https://rs.olm.vn/images/avt/0.png?1311)
\(2022A=2022+2022^2+2022^3+2022^4+...+2022^{2018}\)
\(2021A=2022A-A=2022^{2018}-1\Rightarrow A=\dfrac{2022^{2018}-1}{2021}\)
\(\Rightarrow A< B\)
\(A=\dfrac{3^{2022}+2}{3^{2022}-1}=\dfrac{3^{2022}-1+3}{3^{2022}-1}=1+\dfrac{3}{3^{2022}-1}\)
\(B=\dfrac{3^{2022}}{3^{2022}-3}=\dfrac{3^{2022}-3+3}{3^{2022}-3}=1+\dfrac{3}{3^{2022}-3}\)
Vì \(3^{2022}-1>3^{2022}-3\)
nên \(\dfrac{3}{3^{2022}-1}< \dfrac{3}{3^{2022}-3}\)
=>\(1+\dfrac{3}{3^{2022}-1}< 1+\dfrac{3}{3^{2022}-3}\)
=>A<B