Cho x=15/a-17 (a thuộc z). Xác định a để:
a) x thuộc Q b) x thuộc Q dương
c) x thuộc Q âm d) x = -1
e) x> 1 f) 0 < x < 1
Cho x=15/a-17 (a thuộc z). Xác định a để: a) x thuộc Q b) x thuộc Q dương c) x thuộc Q âm d) x = -1 e) x> 1
By Melody
Đáp án:
$\begin{array}{l}
a)x \in Q\\
\Rightarrow a – 17 \ne 0\\
\Rightarrow a \ne 17\\
b)\left\{ \begin{array}{l}
x \in Q\\
x > 0
\end{array} \right. \Rightarrow \left\{ \begin{array}{l}
a \ne 17\\
\dfrac{{15}}{{a – 17}} > 0
\end{array} \right.\\
\Rightarrow \left\{ \begin{array}{l}
a \ne 17\\
a – 17 > 0
\end{array} \right.\\
\Rightarrow a > 17\\
c)\left\{ \begin{array}{l}
x \in Q\\
x < 0
\end{array} \right.\\
\Rightarrow \left\{ \begin{array}{l}
a \ne 17\\
a – 17 < 0
\end{array} \right.\\
\Rightarrow a < 17\\
d)x = – 1\\
\Rightarrow \dfrac{{15}}{{a – 17}} = – 1\\
\Rightarrow a – 17 = – 15\\
\Rightarrow a = 2\left( {tmdk} \right)\\
e)x > 1\\
\Rightarrow \dfrac{{15}}{{a – 17}} > 1\\
\Rightarrow \dfrac{{15}}{{a – 17}} – 1 > 0\\
\Rightarrow \dfrac{{15 – a + 17}}{{a – 17}} > 0\\
\Rightarrow \dfrac{{32 – a}}{{a – 17}} > 0\\
\Rightarrow \dfrac{{a – 32}}{{a – 17}} < 0\\
\Rightarrow 17 < a < 32\\
f)0 < x < 1\\
\Rightarrow 0 < \dfrac{{15}}{{a – 17}} < 1\\
\Rightarrow \left\{ \begin{array}{l}
a – 17 > 0\\
\dfrac{{15}}{{a – 17}} – 1 < 0
\end{array} \right.\\
\Rightarrow \left\{ \begin{array}{l}
a > 17\\
\dfrac{{15 – a + 17}}{{a – 17}} < 0
\end{array} \right.\\
\Rightarrow \left\{ \begin{array}{l}
a > 17\\
\dfrac{{32 – a}}{{a – 17}} < 0
\end{array} \right.\\
\Rightarrow \left\{ \begin{array}{l}
a > 17\\
32 – a < 0
\end{array} \right.\\
\Rightarrow \left\{ \begin{array}{l}
a > 17\\
a > 32
\end{array} \right.\\
\Rightarrow a > 32
\end{array}$