\(4\left(2x+7\right)^2-9\left(x+3\right)^2=0\)
\(\Leftrightarrow16x^2+112x+196-9x^2-54x-81=0\)
\(\Leftrightarrow7x^2+58x+115=0\)
\(\Leftrightarrow7x^2+23x+35x+115=0\)
\(\Leftrightarrow x\left(7x+23\right)+5\left(7x+23\right)=0\)
\(\Leftrightarrow\left(x+5\right)\left(7x+23\right)=0\)
\(\Leftrightarrow\left[{}\begin{matrix}x+5=0\Leftrightarrow x=-5\\7x+23=0\Leftrightarrow x=-\dfrac{23}{7}\end{matrix}\right.\)
Vậy \(S=\left\{-5;-\dfrac{23}{7}\right\}\)
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