\(=4x^2-28x-8x+56=4x\left(x-7\right)-8\left(x-7\right)=4\left(x-2\right)\left(x-7\right)\)

2) \(x^4-5x^2+4\)

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

\(=x^2\left(x^2-1\right)-4\left(x^2-1\right)\)

\(=\left(x^2-1\right)\left(x^2-4\right)\)

\(=\left(x-1\right)\left(x+1\right)\left(x-2\right)\left(x+2\right)\)

\(81\left(y-4\right)^2-9x^2-36x-36=9\left\{9\left(y-4\right)^2-\left(x+4\right)^2\right\}=9\left\{3\left(y-4\right)-\left(x+4\right)\right\}\left[3\left(y-4\right)+\left(x+4\right)\right]\)

\(=9\left(3y+12-x-4\right)\left(3y+12+x+4\right)=9\left(3y-x+8\right)\left(3y+x+16\right)\)

bạn dùng phuong phap nham ngiệm đi

4x² - 36x + 56 

= 4(x² - 9x + 14)

= 4(x² - 2x - 7x + 14)

= 4[x(x - 2) - 7(x - 2)]

= 4(x - 2)(x - 7)

\(4x^2\)−36x+56=04x2−36x+56

⇒4(x2−9x+14)=0⇒4(x2−9x+14)

⇒4(x2−7x−2x+14)=0⇒4(x2−7x−2x+14)

⇒4x(x−2)−7(x−2)=0⇒4x(x−2)−7(x−2)

⇒4(x−7)(x−2)=0⇒4(x−7)(x−2)

⇒(x−7)(x−2)=0⇒(x−7)(x−2)

⇒[x−7=0x−2=0⇒[x−7=0x−2=0

⇒x=7;x=2⇒x=7;x=2.

a) \(x^3\left(x^2-7\right)^2-36x=x\left[\left(x^3-7x\right)^2-6^2\right]\)

\(=x\left(x^3-7x-6\right)\left(x^3-7x+6\right)\)

\(x\left[\left(x-3\right)\left(x+1\right)\left(x+2\right)\right].\left[\left(x+3\right)\left(x-2\right)\left(x-1\right)\right]\)

\(=\left(x-3\right)\left(x-2\right)\left(x-1\right).x.\left(x+1\right)\left(x+2\right)\left(x+3\right)\)

b) Không pt được.

c) Không pt được.

\(=x^2-36x+324-268\\ =\left(x-18\right)^2-268\\ =\left(x-18-2\sqrt{67}\right)\left(x-18+2\sqrt{67}\right)\)

Lời giải:
\(x^3(x^2-7)^2-36x=x[x^2(x^2-7)^2-36]\\ =x[(x^3-7x)^2-6^2]=x(x^3-7x-6)(x^3-7x+6)\\ =x[x^2(x-3)+3x(x-3)+2(x-3)][x^2(x-2)+2x(x-2)-3(x-2)]\\ =x(x-3)(x^2+3x+2)(x-2)(x^2+2x-3)\\ =x(x-3)(x+1)(x+2)(x-2)(x-1)(x+3)\)