Xét \(\left(7x+1\right)^2-\left(x+7\right)^2-48\left(x^2-1\right)\)

\(=49x^2+14x+1-x^2-14x-49-48x^2+48\)

\(=0\)

Vậy \(\left(7x+1\right)^2-\left(x+7\right)^2=48\left(x^2-1\right)\)

Bài 1:
\(16x^2-\left(4x-5\right)^2=15\)

\(\Leftrightarrow\left(4x-4x+5\right)\left(4x+4x-5\right)=15\)

\(\Leftrightarrow5\left(8x-5\right)=15\)

\(\Leftrightarrow8x=8\Leftrightarrow x=1\)

Vậy x = 1

Bài 2:

\(VT=\left(7x+1\right)^2-\left(x+7\right)^2\)

\(=\left(7x+1-x-7\right)\left(7x+1+x+7\right)\)

\(=\left(6x-6\right)\left(8x+8\right)\)

\(=48\left(x-1\right)\left(x+1\right)\)

\(=48\left(x^2-1\right)=VP\)

\(\Rightarrowđpcm\)

x²-4x+7 = 0 ⇔ x² -4x + 4 + 3 = 0 

⇔ (x-2)²+3=0 ⇔ (x-2)²=-3 (vô lí)

Vậy pt vô nghiệm

*Chứng minh phương trình \(x^2-4x+7=0\) vô nghiệm

Ta có: \(x^2-4x+7=0\)

\(\Leftrightarrow x^2-4x+4+3=0\)

\(\Leftrightarrow\left(x-2\right)^2+3=0\)

mà \(\left(x-2\right)^2+3\ge3>0\forall x\)

nên \(x\in\varnothing\)(đpcm)

\(\left(7x+1\right)^2-\left(x+7\right)^2=\left(7x+1\right).\left(7x+1\right)-\left(x+7\right)\left(x+7\right)=\left(49x^2+7x+7x+1\right)-\left(x^2+7x+7x+49\right)\)\(=49x^2+14x+1-x^2-7x-7x-49=\left(49x^2-x^2\right)+\left(14x-7x-7x\right)-\left(49-1\right)=48x^2-48=48.\left(x^2-1\right)\)

(7x - 1)² - (x+7)² = 48(x² - 1)

<=> 49x² + 14x + 1 - x² - 14x - 49 = 48x² - 48

<=>48x² - 48 = 48x² - 48

<=> 0x = 0(luôn đúng)

Vậy ĐPCM

dễ vcl còn hỏi

 ban nao giup minh vs mjnh vs

1. a) 7x² - 5x - 2 = 7x² - 7x + 2x - 2 = 7x(x - 1) + 2(x - 1) = (x - 1).(7x + 2)

2. 5(2x - 1)² - 3(2x - 1) = 0

<=> (2x - 1).[5(2x - 1) - 3] = 0

<=> (2x - 1).(10x - 8) = 0

<=> (2x - 1) = 0 hoặc (10x - 8) = 0

<=> x = 1/2 hoặc x = 4/5

3. x² - 4x + 7 = (x² - 4x + 4) + 3 = (x - 2)² + 3

Do: (x - 2)² > hoặc = 0 (với mọi x)

Nên (x - 2)² + 3 > hoặc = 3 (với mọi x)

Hay (x - 2)² + 3 > 0 (với mọi x)  => đpcm

\(7x^2-5x-2\)

\(=7x^2-7x+2x-2\)

\(=7x\left(x-1\right)+2\left(x-1\right)\)

\(=\left(x-1\right)\left(7x+2\right)\)

`4)x^2-5x+6`

`=x^2-2x-3x+6`

`=x(x-2)-3(x-2)=(x-2)(x-3)`

`5)x^2+7x+10`

`=x^2+5x+2x+10`

`=x(x+5)+2(x+5)=(x+5)(x+2)`

`6)x+7\sqrt{x}+10`    `ĐK: x >= 0`

`=(\sqrt{x})^2+5\sqrt{x}+2\sqrt{x}+10`

`=\sqrt{x}(\sqrt{x}+5)+2(\sqrt{x}+5)=(\sqrt{x}+5)(\sqrt{x}+2)`

`7)3x^4+7x^2+4`

`=3x^4+3x^2+4x^2+4`

`=3x^2(x^2+1)+4(x^2+1)=(x^2+1)(3x^2+4)`

`8)x^2-x-2`

`=x^2-2x+x-2`

`=x(x-2)+(x-2)=(x-2)(x+1)`

`9)x^6-x^3-2`

`=x^6+x^3-2x^3-2`

`=x^3(x^3+1)-2(x^3+1)`

`=(x^3+1)(x^3-2)`.

Phân tích đa thức thành nhân tử:

\(M=\left(x-1\right)^3-x\left(x+2\right)^2+7x\left(x+\dfrac{1}{7}\right)\)

\(M=x^3-3x^2+3x-1-x\left(x^2+4x+4\right)+7x^2+x\)

\(M=x^3-2x^2+3x-1-x^3-4x^2-4x+7x^2+x\)

\(M=\left(x^3-x^3\right)+\left(-2x^2-x^2-4x^2+7x^2\right)+\left(3x-4x+x\right)-1\)

\(M=-1\)

Vậy...........

Chúc bạn học tốt!!!

M = \(\left(x-1\right)^3-x\left(x+2\right)^2+7x\left(x+\dfrac{1}{7}\right)\)

M=\(x^3-3x^2+3x-1-x.\left(x^2+4x+4\right)+7x^2+x\)

M = \(x^3-3x^2+3x-1-x^3-4x^2-4x+7x^2+x\)

M = \(-1\)

Vậy...

Ta có:

\(M=\left(x-1\right)^3-x\left(x+2\right)^2+7x\left(x+\dfrac{1}{7}\right)\)

=> \(M=x^3-1-3x\left(x-1\right)-x\left(x^2+4x+4\right)+7x^2+x\)

=> \(M=x^3-1-3x^2+3x-x^3-4x^2-4x+7x^2+x\)

=> \(M=-1\)

=> Biểu thức M không phụ thuộc vào x