Bài 4:
a) Ta có: \(x^2-3x=0\)
\(\Leftrightarrow x\left(x-3\right)=0\)
\(\Leftrightarrow\left[{}\begin{matrix}x=0\\x=3\end{matrix}\right.\)
c) Ta có: \(4x^3-x=0\)
\(\Leftrightarrow x\left(2x-1\right)\left(2x+1\right)=0\)
\(\Leftrightarrow\left[{}\begin{matrix}x=0\\x=\dfrac{1}{2}\\x=-\dfrac{1}{2}\end{matrix}\right.\)
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Bài 3:
c) Ta có: \(C=xy\left(x+y\right)-2x-2y\)
\(=\left(x+y\right)\left(xy-2\right)\)
\(=7\cdot\left(8-2\right)=7\cdot6=42\)
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