\(a,x^2\left(x-5\right)+x^2-4x-5=0\)
\(\Leftrightarrow x^2\left(x-5\right)+x^2+x-5x-5=0\)
\(\Leftrightarrow x^2\left(x-5\right)+x\left(x+1\right)-5\left(x+1\right)=0\)
\(\Leftrightarrow x^2\left(x-5\right)+\left(x-5\right)\left(x+1\right)=0\)
\(\Leftrightarrow\left(x-5\right)\left(x^2-x-1\right)=0\)
\(\Leftrightarrow\left[{}\begin{matrix}x-5=0\\x^2-x-1=0\end{matrix}\right.\)
\(\Leftrightarrow\left[{}\begin{matrix}x=5\\x=\dfrac{1+\sqrt{5}}{2}\end{matrix}\right.\)
\(b,x^6-1=0\)
\(\Leftrightarrow x^6=1\)
\(\Leftrightarrow x=1\)
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