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\(=\dfrac{9}{4}\cdot\dfrac{-2}{7}+\dfrac{9}{4}\cdot\dfrac{9}{7}=\dfrac{9}{4}\left(-\dfrac{2}{7}+\dfrac{9}{7}\right)=\dfrac{9}{4}\)
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\(\dfrac{2x-3}{4}.\dfrac{6}{5}=\dfrac{21}{10}\\ \Leftrightarrow\dfrac{2x-3}{4}=\dfrac{7}{4}\\ \Leftrightarrow2x-3=7\\ \Leftrightarrow2x=10\\ \Leftrightarrow x=5\)
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Mình làm vầy không biết đúng không nha
1-1/2+1/2-1/3+1/3-1/4+...+1/x-1/x+1=2020/2021
1-1/x+1=2020/2021
1/x+1=1-2020/2021
1/x+1=1/2021
x+1=2021
x=2020 Vậy x = 2020
Mình làm vắn tắt nha nếu chưa hiểu hỏi tớ tớ chỉ các làm cho
nhớ like cho me nhé . chúc bn học tốt
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\(S=\dfrac{1}{2^2}+\dfrac{1}{\left(2.2\right)^2}+\dfrac{1}{\left(2.3\right)^2}+...+\dfrac{1}{\left(2.10\right)^2}\)
\(=\dfrac{1}{2^2}+\dfrac{1}{2^2.2^2}+\dfrac{1}{2^2.3^2}+...+\dfrac{1}{2^2.10^2}\)
\(=\dfrac{1}{2^2}\left(1+\dfrac{1}{2^2}+\dfrac{1}{3^2}+...+\dfrac{1}{10^2}\right)\)
\(< \dfrac{1}{2^2}\left(1+\dfrac{1}{1.2}+\dfrac{1}{2.3}+...+\dfrac{1}{9.10}\right)\)
\(=\dfrac{1}{4}\left(1+1-\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{3}+...+\dfrac{1}{9}-\dfrac{1}{10}\right)\)
\(=\dfrac{1}{4}\left(2-\dfrac{1}{10}\right)< \dfrac{1}{4}.2=\dfrac{1}{2}\) (đpcm)
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2. C
3. D
4. A
đập giờ:V