Question

Tìm nghiệm của các đa thức sau:

a) N(x) = (x + 5).(4x² – 1)

b) P(x) = 9x³ – 25x

Answer 1

a/ \(\left(x+5\right)\left(4x^2-1\right)=0\)

\(\Leftrightarrow\left(x+5\right)\left(2x-1\right)\left(2x+1\right)=0\)

\(\Leftrightarrow\left[{}\begin{matrix}x+5=0\\2x-1=0\\2x+1=0\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}x=-5\\x=\dfrac{1}{2}\\x=-\dfrac{1}{2}\end{matrix}\right.\)

Vậy.......................................

b/ \(9x^3-25x=0\)

\(\Leftrightarrow x\left(9x^2-25\right)=0\)

\(\Leftrightarrow x\left(3x-5\right)\left(3x+5\right)=0\)

\(\Leftrightarrow\left[{}\begin{matrix}x=0\\3x-5=0\\3x+5=0\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}x=0\\x=\dfrac{5}{3}\\x=-\dfrac{5}{3}\end{matrix}\right.\)

Vậy................................