Question

Tìm x bt:

a) x³+5x²−4x−20=0

Answer 1

a) x³+5x²−4x−20=0

    x².x+5.x²-4x-4.5=0

    x².(x+5)-4.(x+5)=0

    (x+5).(x²-4)=0

 Ta có:x²-4=(x+2)(x-2)

         Do đó: (x+5).(x²-4)=0

                     (x+5)(x+2)(x-2)=0

\(\Rightarrow\left[\begin{array}{nghiempt}x+5=0\\x+2=0\\x-2=0\end{array}\right.\)\(\Rightarrow\left[\begin{array}{nghiempt}x=-5\\x=-2\\x=2\end{array}\right.\)

                  Vậy x=-5;-2;2

Answer 2

a)x³+5x²-4x-20=0

=>x²(x+5)-4(x+5)=0

=>(x²-4)(x+5)=0

=>(x-2)(x+2)(x+5)=0

=>x-2=0 hoặc x+2=0 hoặc x+5=0

\(\Rightarrow\begin{cases}x=-5\\x=-2\\x=2\end{cases}\)