$\text{#nan}$
$\text{a) }$
$\text{có xy = $x_{1}$ $y_{1}$ = $x_{2}$ $y_{2}$ }$
$\text{mà : $x_{1}$ = 14 ; $x_{2}$ = 21 }$
$\text{=> 14 $y_{1}$ = 21 $y_{2}$ }$
$\text{=> $\frac{y1}{14}$ = $\frac{y2}{21}$}$
$\text{Mà $y_{1}$ - $y_{2}$ = 3 }$
$\text{=> \(\left[ \begin{array}{l}y_{1} = 9\\y_{2} = 6\end{array} \right.\) }$
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$\text{b) }$
$\text{có x và y = $x_{1}$ $y_{1}$ = 14 . 9 = 126}$
$\text{suy ra y = $\frac{126}{x}$ }$