Giải:
a) \(\left(x+8\right)^2=121\)
\(\Leftrightarrow\left[{}\begin{matrix}x+8=-11\\x+8=11\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}x=-19\\x=3\end{matrix}\right.\)
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b) \(x^2+8x+16=0\)
\(\Leftrightarrow x^2+2.4.x+4^2=0\)
\(\Leftrightarrow\left(x+4\right)^2=0\)
\(\Leftrightarrow x+4=0\)
\(\Leftrightarrow x=-4\)
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c) \(4x^2-12x=-9\)
\(\Leftrightarrow4x^2-12x+9=0\)
\(\Leftrightarrow\left(2x\right)^2-2.2x.3+3^2=0\)
\(\Leftrightarrow\left(2x-3\right)^2=0\)
\(\Leftrightarrow2x-3=0\)
\(\Leftrightarrow x=\dfrac{3}{2}\)
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