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`x^2 + 2x - 3 = 0`

`<=> x^2 - x + 3x - 3 = 0`

`<=> (x^2 - x) + (3x - 3) = 0`

`<=> x(x - 1) + 3(x - 1) = 0`

`<=>` \(\left[ \begin{array}{l}x + 3=0\\x-1 = 0\end{array} \right.\)

`<=>` \(\left[ \begin{array}{l}x=-3\\x=1 \end{array} \right.\) 

Vậy `x ∈ { -3; 1}` thì `x^2 + 2x - 3 = 0`