x3 - 19x - 30 = x3 + 2x2 - 2x2 - 4x - 15x - 30
= (x3 + 2x2) - (2x2 + 4x) - (15x + 30)
= x2( x + 2) - 2x( x + 2) - 15( x + 2)
= (x+2).(x2 - 2x - 15)
= (x + 2)(x2+ 3x - 5x - 15)
= ( x+ 2).\(\left[\text{x(x+3) - 5 (x+3)}\right]\)
= (x+2)(x+3)(x-5)
Chúc bạn học tốt!
\(=x^3-25x+6x-30\)
\(=x\left(x-5\right)\left(x+5\right)+6\left(x-5\right)\)
\(=\left(x-5\right)\left(x^2+5x+6\right)\)
\(\Leftrightarrow\left(x-5\right)\left[x\left(x+2\right)+3\left(x+2\right)\right]\)
\(=\left(x-5\right)\left(x+2\right)\left(x+3\right)\)
