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a/ \(\dfrac{x-7}{2}< 0\Rightarrow x-7< 0\Rightarrow x< 7\)
Vậy........
b/ \(\dfrac{x+3}{x+5}< 0\)
\(\Rightarrow\left[{}\begin{matrix}\left\{{}\begin{matrix}x+3< 0\\x+5>0\end{matrix}\right.\\\left\{{}\begin{matrix}x+3>0\\x+5< 0\end{matrix}\right.\end{matrix}\right.\)\(\Rightarrow\left[{}\begin{matrix}\left\{{}\begin{matrix}x< -3\\x>-5\end{matrix}\right.\\\left\{{}\begin{matrix}x>-3\\x< -5\end{matrix}\right.\end{matrix}\right.\)
\(\Rightarrow-5< x< -3\)
Vậy........
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A = \(\dfrac{2}{3.5}\) + \(\dfrac{2}{5.7}\) + \(\dfrac{2}{7.9}\) + ... + \(\dfrac{2}{19.21}\)
A = \(\dfrac{1}{3}\) - \(\dfrac{1}{5}\) + \(\dfrac{1}{5}\) - \(\dfrac{1}{7}\) + \(\dfrac{1}{7}\) - \(\dfrac{1}{9}\) + ... + \(\dfrac{1}{19}\) - \(\dfrac{1}{21}\)
A = \(\dfrac{1}{3}\) - \(\dfrac{1}{21}\)
A = \(\dfrac{2}{7}\)
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1,
\(\frac{25}{12}+\left(\frac{-4}{12}\right)=\frac{7}{4}\)
\(\frac{-10}{8}+\frac{15}{4}=\frac{5}{2}\)
\(\frac{3}{8}+\frac{-14}{6}=\frac{-47}{24}\)
\(\frac{350}{150}+\left(\frac{-200}{360}\right)=\frac{16}{9}\)
\([\frac{5}{8}+\left(\frac{-3}{4}\right)]+\frac{15}{6}=\frac{-1}{8}+\frac{15}{6}=\frac{19}{8}\)
\(\frac{7}{3}+[\left(\frac{-5}{6}\right)+\left(\frac{-2}{3}\right)]=\frac{7}{3}+\left(\frac{-3}{2}\right)=\frac{5}{6}\)
(x-2/5) . (x + 2/7) >0
x - 2/5 > 0 và x + 2/7 >0 Hoặc x - 2/5 < 0 và x + 2/7 <0
x > 2/5 và x > - 2/7 Hoặc x < 2/5 và x < - 2/7
x > 2/5 hoặc x < -2/7