Question

Giải phương trình sau

x³ + 27 + (x + 3)( x – 9) = 0     
f) x³ – 7x² = - 6x

g) (x + 1)(x + 2)(x + 4)(x + 5) = 4   

Answer 1

a: \(\Leftrightarrow\left(x+3\right)\left(x^2-3x+9\right)+\left(x+3\right)\left(x-9\right)=0\)

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

hay \(x\in\left\{-3;0;2\right\}\)

b: \(\Leftrightarrow x^3-7x^2+6x=0\)

\(\Leftrightarrow x\left(x^2-7x+6\right)=0\)

\(\Leftrightarrow x\left(x-1\right)\left(x-6\right)=0\)

hay \(x\in\left\{0;1;6\right\}\)

g: \(\Leftrightarrow\left(x^2+6x+5\right)\left(x^2+6x+8\right)-4=0\)

\(\Leftrightarrow\left(x^2+6x\right)^2+13\left(x^2+6x\right)+36=0\)

\(\Leftrightarrow\left(x^2+6x+4\right)\left(x^2+6x+9\right)=0\)

hay \(x\in\left\{-3;-3+\sqrt{5};-3-\sqrt{5}\right\}\)