Bài 20 : 
1) x^2-1 =x^2-1^2= (x-1)(x+1)
2) x^2-25 = x^2-5^2=(x-5)(x+5)
3) x^2-16 =x^2-4^2=(x-4)(x+4)
4) x^2-9=x^2-3^2=(x-3)(x+3)
5)x^2-m^2=(x-m)(x+m)
6)4x^2-y^2=(2x)^2-y^2=(2x-y)(2x+y)

Mk bổ sung thêm cho dễ hiểu nhé

1) x^2-1 = (x-1)(x+1)
2) x^2-25 = (x-5)(x+5)
3) x^2-16 =(x-4)(x+4)
4) x^2-9=(x-3)(x+3)
5)x^2-m^2=(x-m)(x+m)
6)4x^2-y^2=(2x-y)(2x+y)

Bài 21 : 
a) x^3+3x^2+3x+1
= x^3+3.x^2.1+3.x.1^2+1^3
=(x+1)^3
b) x^3+6x^2+12x+8
=x^3+3.x^2.2+3.x.2^2+2^3
=(x+2)^3
c) 8x^3+12x^2y+6xy^2+y^3
=(2x)^3 + 3.(2x)^2.y+3.2x.y^2+y^3
=(2x+y)^3
d) x^3+15x^2+75x+125
=x^3+3.x^2.5+3.x.5^2+5^3
=x+5)^3

1) x^2-1 = (x-1)(x+1)
2) x^2-25 = (x-5)(x+5)
3) x^2-16 =(x-4)(x+4)
4) x^2-9=(x-3)(x+3)
5)x^2-m^2=(x-m)(x+m)
6)4x^2-y^2=(2x-y)(2x+y)

Bài 20 :
$$\begin{gathered} 1)x^2-1 \\ =(x-1\left)\right(x+1\left) \\ 2\right)x^2-25 \\ =(x-5)(x+5) \\ 3)x^2-16 \\ =(x-4\left)\right(x+4\left) \\ 4\right)x^2-9 \\ =(x-3)(x+3) \\ 5)x^2-m^2 \\ =(x-m\left)\right(x+m\left) \\ 6\right)4x^2-y^2 \\ \mathit{=}{\mathit{\left(}\mathit{2}x\mathit{\right)}}^{\mathit{2}}\mathit{-}{y}^2 \\ =(2x-y)(2x+y) \end{gathered}$$