Question

\(3x^3=21x\)

Tim x

Answer 1

\(3x^3=21x\)

=> \(x^2=\frac{21x}{3x}\)

=> \(x^2=7\)

=> \(\orbr{\begin{cases}x=\sqrt{7}\\x=-\sqrt{7}\end{cases}}\)

Answer 2

\(3x^3-21x=0\)

\(3x.\left(x^2-7\right)=0\)

\(\Rightarrow\orbr{\begin{cases}3x=0\\x^2=7\end{cases}\Rightarrow\orbr{\begin{cases}x=0\\x=\pm\sqrt{7}\end{cases}}}\)

Vậy x=0 hay x=\(\pm\sqrt{7}\)