`f(x)=2x^4-9x^3-18x^2+71x-30=0`
`⇒(2x^4-4x^3)-(5x^3-10x^2)-(28x^2-56x)+(15x-30)=0`
`⇒2x^3(x-2)-5x^2(x-2)-28x(x-2)+15(x-2)=0`
`⇒(x-2)(2x^3-5x^2-28x+15)=0`
`⇒(x-2)[(2x^3+6x^2)-(11x^2+33x)+(5x+15)]=0`
`⇒(x-2)[2x^2(x+3)-11x(x+3)+5(x+3)]=0`
`⇒(x-2)(x+3)(2x^2-11x+5)=0`
`⇒(x-2)(x+3)[(2x^2-10x)-(x-5)]=0`
`⇒(x-2)(x+3)[2x(x-5)-(x-5)]=0`
`⇒(x-2)(x+3)(x-5)(2x-1)=0`
\(⇒\left[ \begin{array}{l}x-2=0\\x+3=0\\x-5=0\\2x-1=0\end{array} \right.\)
\(⇒\left[ \begin{array}{l}x=2\\x=-3\\x=5\\x=\dfrac{1}{2}\end{array} \right.\)
Vậy nghiệm của `f(x)` là `2; -3; 5; 1/2`
