(x² + 6x + 5)(x² - 10x + 21) - 20

= (x² + x + 5x + 5)(x² - 3x - 7x + 21)  - 20

= (x + 1)(x + 5)(x - 3)(x - 7) - 20

= (x² -2x - 3)(x² - 2x- 35) - 20

Đặt x² - 2x - 19 = a

=> (a + 16)(a - 16) - 20 = a² - 256 - 20 = a² - 276

= \(\left(a-2\sqrt{69}\right)\left(a+2\sqrt{69}\right)\)

= \(\left(a^2-2x-19-2\sqrt{69}\right)\left(x^2-2x-19+2\sqrt{69}\right)\)

c ) =x²-x-9x+9

=x(x-1)-9(x-1)

=(x-1)(x-9)

Đặt x²+10x+5=a.Ta có biểu thức là : a(a+8)+16=a²+8a+16=(a+4)²=(x²+10x+9)²

\(=x^2+2x+8x+16\)

\(=x\cdot\left(x+2\right)+8\cdot\left(x+2\right)\)

\(=\left(x+2\right)\cdot\left(x+8\right)\)

`#3107.101107`

`x^2 - y^2 + 10x - 10y`

`= (x^2 - y^2) + (10x - 10y)`

`= (x - y)(x + y) + 10(x - y)`

`= (x + y + 10)(x - y)`

_____

Sử dụng HĐT:

`A^2 - B^2 = (A - B)(A + B).`

\(x^2-y^2+7x-7y=\left(x^2-y^2\right)+\left(7x-7y\right)=\left(x-y\right)\left(x+y\right)+7\left(x-y\right)=\left(x-y\right)\left(x+y+7\right)\)

\(x^2-10x+25-9y^2=\left(x^2-10x+25\right)-\left(3y\right)^2=\left(x-5\right)^2-\left(3y\right)^2=\left(x-3y-5\right)\left(x+3y-5\right)\)

\(x^2-y^2+7x-7y=\left(x-y\right)\left(x+y\right)+7\left(x-y\right)=\left(x-y\right)\left(x+y+7\right)\)

\(x^2-10x+25-9y^2=\left(x-5\right)^2-\left(3y\right)^2=\left(x-5-3y\right)\left(x-5+3y\right)\)

1. x² - 10x + 25 - 9y²

<=> x² - 10x + 5² - (3y)²

<=> (x - 5)² - (3y)²

<=> (x - 5 - 3y)(x - 5 + 3y)

2. x² - y² + 7x - 7y

<=> (x - y)(x + y) + 7(x - y)

<=> (x + y + 7)(x - y)

x³ - 7x² + 10x

=x³ - 2x² - 5x² + 10x

=(x³ -2x²) - (5x² - 10x)

= x²( x - 2) - 5x( x - 2)

= (x - 2) (x² - 5x)

Gõ trên máy tính nên hơi lâu 

Cảm ơn mình đê

-x²+10x-25=-(x²-10x+25)=-(x²-2.5+25)=-(x-5)²

a) \(-10x^2-17x+6\)

\(=-10x^2-20x+3x+6\)

\(=-10x\left(x+2\right)+3\left(x+2\right)\)

\(=\left(x+2\right)\left(3-10x\right)\)

b)\(x^2-10x+9\)

\(=x^2-x-9x+9\)

\(=x\left(x-1\right)-9\left(x-1\right)\)

\(=\left(x-1\right)\left(x-9\right)\)

c) \(x^2-10x+24\)

\(=x^2-4x-6x+24\)

\(=x\left(x-4\right)-6\left(x-4\right)\)

\(=\left(x-4\right)\left(x-6\right)\)