\(\left(x^2-4x\right)-8\left(x^2-4x\right)+15=0\)
\(\Rightarrow-7\left(x^2-4x\right)=-15\)
\(\Rightarrow-7\left(x^2-4x\right)+28-28=-15\)
\(\Rightarrow-7\left(x^2-4x+4\right)+28=-15\)
\(\Rightarrow-7\left(x-2\right)^2=-43\)
\(\Rightarrow\left(x-2\right)^2=\dfrac{43}{7}\)
\(\Rightarrow\left[{}\begin{matrix}x-2\approx\dfrac{62}{25}\\x-2\approx-\dfrac{62}{25}\end{matrix}\right.\)
\(\Rightarrow\left[{}\begin{matrix}x\approx\dfrac{112}{25}\\x\approx-\dfrac{12}{25}\end{matrix}\right.\)
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