Giup mik voi mai mik nop roi pls???????? Ai nhanh nhat 5*+cau tra loi hay nhat???????????? Tim so nguyen x y thoa man A)2/n-1 b)2/-n+2. C)-3/2n-1 A)x/2=y/5 v

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1. Đáp án:

   Áp dụng tính chất dãy tỉ số bằng nhau ta có:

   $\begin{array}{l}
   a)\dfrac{2}{{n – 1}} \in Z\\
    \Rightarrow \left( {n – 1} \right) \in \left\{ { – 2; – 1;1;2} \right\}\\
    \Rightarrow n \in \left\{ { – 1;0;2;3} \right\}\\
   b)\dfrac{2}{{ – n + 2}} \in Z\\
    \Rightarrow \left( { – n + 2} \right) \in \left\{ { – 2; – 1;1;2} \right\}\\
    \Rightarrow  – n \in \left\{ { – 4; – 3; – 1;0} \right\}\\
    \Rightarrow n \in \left\{ {0;1;3;4} \right\}\\
   c)\dfrac{{ – 3}}{{2n – 1}} \in Z\\
    \Rightarrow 2n – 1 \in \left\{ { – 3; – 1;1;3} \right\}\\
    \Rightarrow 2n \in \left\{ { – 2;0;2;4} \right\}\\
    \Rightarrow n \in \left\{ { – 1;0;1;2} \right\}\\
   a)\dfrac{x}{2} = \dfrac{y}{5} = \dfrac{{x + y}}{{2 + 5}} = \dfrac{{35}}{7} = 5\\
    \Rightarrow \left\{ \begin{array}{l}
   x = 10\\
   y = 25
   \end{array} \right.\\
   b)\dfrac{{x + 2}}{{y + 10}} = \dfrac{1}{5}\\
    \Rightarrow \dfrac{{x + 2}}{1} = \dfrac{{y + 10}}{5} = \dfrac{{3x + 6}}{3}\\
    = \dfrac{{y + 10 – 3x – 6}}{{5 – 3}} = \dfrac{{\left( {y – 3x} \right) + 4}}{2}\\
    = \dfrac{{2 + 4}}{2} = 3\\
    \Rightarrow \left\{ \begin{array}{l}
   x + 2 = 3\\
   y + 10 = 15
   \end{array} \right.\\
    \Rightarrow \left\{ \begin{array}{l}
   x = 1\\
   y = 5
   \end{array} \right.\\
   Vậy\,x = 1;x = 5\\
   c)\dfrac{x}{4} = \dfrac{y}{5} = \dfrac{{2x}}{8} = \dfrac{{2x – y}}{{8 – 5}} = \dfrac{{15}}{3} = 5\\
    \Rightarrow \left\{ \begin{array}{l}
   x = 20\\
   y = 25
   \end{array} \right.
   \end{array}$

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