x^3-19x-30
=x^3-25x+6x-30
=x(x^2-25)+6(x-5)
=x(x+5)(x-5)+6(x-5)
=(x-5)(x^2+5x+6)
=(x-5)(x^2+2x+3x+6)
=(x-5)[x(x+2)+3(x+2)]
=(x-5)(x+2)(x+3)
\(x^3-19x-30=x^3+2x^2-2x^2-4x-15x-30\)
\(\Rightarrow x^2\left(x+2\right)-2x\left(x+2\right)-15\left(x+2\right)\)
\(\Rightarrow\left(x+2\right)\left(x^2-2x-15\right)\)
\(\Rightarrow\left(x+2\right)\left(x+3\right)\left(x-5\right)\)
x^3 + 3x^2 - 3x^2 - 9x - 10x - 30
= x^2 ( x+3) - 3x ( x +3) -10(x+3)
=(x^2 -3x - 10)(x+3)
=(x^2 - 5x + 2x -10)(x+3)
=[x(x-5)+2(x-5)](x+3)
=(x-5)(x+2)(x+3)
@Lê Anh Tú, dấu "=" chứ ko phải dấu suy ra "=>" đâu nhé bạn ^^
