Đáp án:

Giải thích các bước giải:

`1)`

 `x^2+5x-14=0`

`=>x^2+7x-2x-14=0`

`=>x.(x+7)-2.(x+7)=0`

`=>(x-2).(x+7)=0`

`=>`\(\left[ \begin{array}{l}x=2\\x=-7\end{array} \right.\) 

`2)`

`x^4+8x^2-9=0`

đặt `x^2=a(a>=0)`

`=>``a^2+8x-9=0`

có `a+b+c=0`

`=>`\(\left[ \begin{array}{l}a_1=1(TM)\\a_2=-9(KTM)\end{array} \right.\) 

`=>x^2=a=1`

`=>x=+-1`

`3)`

$\left \{ {{2x-3y=7} \atop {x+2y=7}} \right.$

`=>`$\left \{ {{4x-6y=14} \atop {3x+6y=21}} \right.$ 

`=>`$\left \{ {{7x=35} \atop {3x+6y=21}} \right.$ 

`=>`$\left \{ {{x=5} \atop {6y=21-15}} \right.$ 

`=>`$\left \{ {{x=5} \atop {6y=6}} \right.$ 

`=>`$\left \{ {{x=5} \atop {y=1}} \right.$ 

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