Question

Tìm x, biết :

1) x² + 8x + 7 = 0

2) 4x² + 12x + 6 = 0

Answer 1

1) \(x^2+8x+7=0\)

\(\Leftrightarrow x^2+7x+x+7=0\)

\(\Leftrightarrow x\left(x+7\right)+\left(x+7\right)=0\)

\(\Leftrightarrow\left(x+7\right)\left(x+1\right)=0\)

\(\Leftrightarrow\left[{}\begin{matrix}x=-7\\x=-1\end{matrix}\right.\)

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2) \(4x^2+12x+6=0\)

\(\Leftrightarrow4\left(x^2+3x+\frac{3}{2}\right)=0\)

\(\Leftrightarrow x^2+2\cdot x\cdot\frac{3}{2}+\frac{9}{4}-\frac{3}{4}=0\)

\(\Leftrightarrow\left(x+\frac{3}{2}\right)^2=\frac{3}{4}=\left(\frac{\pm\sqrt{3}}{2}\right)^2\)

\(\Leftrightarrow x=\frac{\pm\sqrt{3}-3}{2}\)

Vậy...