a) Ta có: \(7x\left(x-20\right)-x+20=0\)
\(\Leftrightarrow\left(x-20\right)\left(7x-1\right)=0\)
\(\Leftrightarrow\left[{}\begin{matrix}x=20\\x=\dfrac{1}{7}\end{matrix}\right.\)
b) Ta có: \(x^3-15x=0\)
\(\Leftrightarrow x\left(x-\sqrt{15}\right)\left(x+\sqrt{15}\right)=0\)
\(\Leftrightarrow\left[{}\begin{matrix}x=0\\x=\sqrt{15}\\x=-\sqrt{15}\end{matrix}\right.\)