Question

tìm x, biết:  x^3 + x^2 -36=0

Answer 1

\(x^3+x^2-36=0\)

\(\Leftrightarrow\left(x^3-3x^2\right)+\left(4x^2-12x\right)+\left(12x-36\right)=0\)

\(\Leftrightarrow\left(x-3\right)\left(x^2+4x+12\right)=0\)

Mà \(x^2+4x+12=\left(x^2+4x+4\right)+8\)

                                  \(=\left(x+2\right)^2+8>0\)\(\forall x\)

\(\Rightarrow x-3=0\)

\(\Rightarrow x=3\)

Vậy\(x=3\)