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\(A>\frac{1}{1.2}+\frac{1}{3.2}+\frac{1}{4.3}+..+\frac{1}{9.10}=\frac{1}{1}-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{9}-\frac{1}{10}\)
\(=1-\frac{1}{10}>1\)
=> A>1
\(-4\frac{1}{3}.\left(\frac{1}{2}-\frac{1}{6}\right)\le x\le\frac{2}{3}.\left(\left|-\frac{1}{3}\right|-\left|-\frac{1}{2}\right|-\left|-\frac{3}{-4}\right|\right)\)
\(\Leftrightarrow-\frac{13}{3}.\frac{1}{3}\le x\le\frac{2}{3}.\left(\frac{1}{3}-\frac{1}{2}-\frac{3}{4}\right)\)
\(\Leftrightarrow-\frac{13}{9}\le x\le\frac{2}{4}.-\frac{11}{12}\)
\(\Leftrightarrow-\frac{13}{9}\le x\le-\frac{11}{24}\)
\(\Rightarrow x\in\left\{-1,0\right\}\) ( do \(x\in Z\) )
Vậy : \(x\in\left\{-1,0\right\}\)
\(-4\frac{1}{3}\left(\frac{1}{2}-\frac{1}{6}\right)\le x\le\frac{2}{3}\left(|\frac{-1}{3}|-|\frac{-1}{2}|-|\frac{-3}{-4}|\right)\)
\(\Rightarrow\frac{-13}{9}\le x\le\frac{-11}{18}\)
\(\Rightarrow x\in\left[\frac{-13}{9};\frac{-11}{18}\right]\)
Ta có :
\(\frac{1}{1^2}< \frac{1}{1\cdot2};\frac{1}{2^2}< \frac{1}{2\cdot3};.....;\frac{1}{50^2}< \frac{1}{49\cdot50}\)
\(\Rightarrow\frac{1}{1^2}+\frac{1}{2^2}+...+\frac{1}{50^2}< \frac{1}{1\cdot2}+\frac{1}{2\cdot3}+...+\frac{1}{49\cdot50}\)
\(\Rightarrow a< 1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{49}-\frac{1}{50}\)
\(\Rightarrow a< 1-\frac{1}{50}=\frac{49}{50}\)
\(a< \frac{49}{50}< 1< 2\)
\(\Rightarrow a< 2\)
thanks bạn rất nhiều