Question

Tìm x biết:

a) x³ - 6x² + 12x - 8 = 0

b)x³ + 9x² +27x + 27 = 0

Answer 1

a ) \(x^3-6x^2+12x-8=0\)

\(\Leftrightarrow x^3-3.x^2.2+3.x.2^2-2^3=0\)

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

\(\Leftrightarrow\left(x-2\right)=0\)

\(\Leftrightarrow x=2\)

b ) \(x^3+9x^2+27x+27=0\)

\(\Leftrightarrow x^3+3.x^2.3+3.x.3^2+3^3=0\)

\(\Leftrightarrow\left(x-3\right)^3=0\)

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

\(\Leftrightarrow x=3\)

 

Answer 2

a) x³ - 6x² + 12x - 8 = 0

   ( x - 2 ) ³                = 0

    x - 2                      = 0

    x                           = 2

b) x³ + 9x² + 27x + 27 = 0

    ( x + 3 )³                    = 0

      x + 3                         = 0

      x                                = -3