Ta có :
`x^3 + (a+b+c)x^2 + (ab+bc+ca)x + abc`
`= x^3 + ax^2 + bx^2 + cx^2 + abx + bcx + acx + abc`
` = (x^3 + cx^2) + (bx^2 + bcx) + (ax^2 + acx) + (abx + abc)`
`= x^2 (x+c) + bx (x+c) + ax (x+c) + ab (x+c)`
`= (x^2 + bx + ax + ab)(x+c)`
` = [ x(x+b) + a(x+b)] (x+c)`
`= (x+a)(x+b)(x+c)`
Vậy `(x+a)(x+b)(x+c) = x^3 + (a+b+c)x^2 + (ab+bc+ca)x + abc (đpcm)`.