3x = -4y và x + y =10 ( 2/5 - 3x ) ^^2 = 9/25

$3x$ = $-4y$ và $x$ + $y$ = $10$

$\Longrightarrow$ `x/-4` = `y/3`

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

`x/-4` = `y/3` = $\dfrac{x + y}{-4 + 3}$ = `10/-1` = $-10$

$\Rightarrow$ `x/-4` = $-10$                 $\Longrightarrow$ $x$ = $-10$ . $4$ = $-40$

$\Rightarrow$ `y/3` = $-10$                   $\Longrightarrow$ $y$ = $-10$ . $3$ = $-30$

Vậy $x$ = $-40$

$y$ = $-30$ 

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( `2/5` - $3x$ )$^2$ = `9/25`

$\Rightarrow$ ( `2/5` - $3x$ )$^2$ = ( `3/5` )$^2$ 

$\Rightarrow$ `2/5` - $3x$ = $\pm$`3/5`

$\Rightarrow$ $\left[\begin{matrix} 2/5-3x=3/5\\ 2/5-3x=-3/5\end{matrix}\right.$

$\Rightarrow$ $\left[\begin{matrix} 3x=2/5-3/5\\ 3x=2/5+3/5\end{matrix}\right.$

$\Rightarrow$ $\left[\begin{matrix} 3x=-1/5\\ 3x=1\end{matrix}\right.$

$\Rightarrow$ $\left[\begin{matrix} x=-1/5:3\\ x=1:3\end{matrix}\right.$

$\Longrightarrow$ $\left[\begin{matrix} x=-1/15\\ x=1/3\end{matrix}\right.$

Vậy $x$ $\in$ `{`-1/15` ;  `1/3`}`