\(x^3+5x^2-4x-20=0\)
\(\Leftrightarrow\left(x^3+5x^2\right)-\left(4x+20\right)=0\)
\(\Leftrightarrow x^2\left(x+5\right)-4\left(x+5\right)=0\)
\(\Leftrightarrow\left(x+5\right)\left(x^2-4\right)=0\)
\(\Leftrightarrow\left(x+5\right)\left(x-2\right)\left(x+2\right)\)
\(\Leftrightarrow\left[\begin{array}{nghiempt}x+5=0\\x-2=0\\x+2=0\end{array}\right.\Leftrightarrow\left[\begin{array}{nghiempt}x=-5\\x=2\\x=-2\end{array}\right.\)
a)x3+5x2-4x-20=0
x2.x+5.x2-4.(x+5)=0
x2.(x+5)-4.(x+5)=0
(x+5).(x2-4)=0
\(\Rightarrow\left[\begin{array}{nghiempt}x+5=0\\x^2-4=0\end{array}\right.\)\(\Rightarrow\left[\begin{array}{nghiempt}x=-5\\x^2=4\end{array}\right.\)\(\Rightarrow\left[\begin{array}{nghiempt}x=-5\\x=2;-2\end{array}\right.\)