\(a,5x\left(x+4\right)-x-4=0\\ \Leftrightarrow5x\left(x+4\right)-\left(x+4\right)=0\\ \Leftrightarrow\left(x+4\right)\left(5x-1\right)=0\\ \Leftrightarrow\left[{}\begin{matrix}x=-4\\x=\dfrac{1}{5}\end{matrix}\right.\\ b,x^2-8x=0\\ \Leftrightarrow\left(x-9\right)\left(x+9\right)=0\\ \Leftrightarrow\left[{}\begin{matrix}x=9\\x=-9\end{matrix}\right.\\ c,\left(x-1\right)\left(x+1\right)-x\left(x-4\right)=15\\ \Leftrightarrow x^2-1-x^2+4x=15\\ \Leftrightarrow4x=16\\ \Leftrightarrow x=4\)
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