\(x^2 - 3x - 18\)
\(= x^2 + 3x - 6x - 18\)
\(= x(x + 3) - 6(x+3) \)
\(= (x+3)(x-6)\)
Cách 1 : \(x^2-3x-18\)
\(=x^2-2x.\dfrac{3}{2}+\dfrac{9}{4}-\dfrac{81}{4}\)
\(=\left(x-\dfrac{3}{2}\right)^2-\left(\dfrac{9}{2}\right)^2\)
\(=\left(x-\dfrac{3}{2}-\dfrac{9}{2}\right)\left(x-\dfrac{3}{2}+\dfrac{9}{2}\right)\)
\(=\left(x-\dfrac{3+9}{2}\right)\left(x+\dfrac{9-3}{2}\right)\)
\(=\left(x-6\right)\left(x+3\right)\)
Cách 2 : \(x^2-3x-18\)
\(=x^2-6x+3x-18\)
\(=x\left(x-6\right)+3\left(x-6\right)\)
\(=\left(x-6\right)\left(x+3\right)\)
x2-3x-18 = x2+3x-6x-18 = x(x+3)-6(x+3)
= (x+3)(x-6)
