x^2+4x+3=x^2+x+3x+3=x(x+1)+3(x+1)=(x+1)(x+3
x^2+5x+6=x^2+2x+3x+6=x(x+2)+3(x+2)=(x+2)(x+3

Phân tích đa thức thành nhân tử:
a) x^2 + 4x + 3
= x^2 + (3x + x) + 3
= (x^2 + 3x) + (x + 3)
= x(x + 3) + (x + 3)
= (x + 3)(x + 1)

*Cách khác:
x^2 + 4x + 3
= x^2 + (3x + x) + 3
= (x^2 + x) + (3x + 3)
= x(x + 1) + 3(x + 1)
​= (x + 3)(x + 1)
 

b) x^2 + 5x + 6
= x^2 + (3x + 2x) + 6
= (x^2 + 3x) + (2x + 6)
= x(x + 3) + 2(x + 3)
= (x + 3)(x + 2)

* Cách khác:
x^2 + 5x + 6
= x^2 + (3x + 2x) + 6
= (x^2 + 2x) + (3x + 6)
= x(x + 2) + 3(x + 2)
​= (x + 3)(x + 2)
 

a) x^2 + 4x + 3
= x^2 + 3x + x + 3
= x(x + 3) + (x + 3)
= (x + 3)(x + 1)
b) x^2 + 5x + 6
= x^2 + 3x + 2x + 6
= x(x + 3) + 2(x + 3)
= (x + 3)(x + 2)

Đề 
= x^2 + 4x + 4 - 1
= ( x + 2 )^2 - 1^2
= ( x+2+1)(x+2-1)
= ( x+3)(x+1)