Ta có \(P=2x^2-4x+7=2\left(x^2-2x+\dfrac{7}{2}\right)=2\left(x^2-2x+1+\dfrac{5}{2}\right)\) \(=2\left(x-1\right)^2+5\)

Mà \(2\left(x-1\right)^2\ge0\Leftrightarrow2\left(x-1\right)^2+5\ge5>0\) hay \(P>0\) (đpcm)

Mặt khác \(H=-2x^2-16x+38=-2\left(x^2+8x-19\right)\) \(=-2\left(x^2+8x+16-35\right)=-2\left(x+4\right)^2-70\)

Mà \(-2\left(x+4\right)^2\le0\Leftrightarrow-2\left(x+4\right)^2-70\le-70< 0\) nên ta có \(H< 0\) (đpcm)

`2x^2 - 4x + 7`

`<=> 2x^2 - 4x  + 2 + 5`

`<=> 2.(x-1)^2 + 5`

Mà : \(2\left(x-1\right)^2\ge0\forall x\)

`=>` \(2\left(x-1\right)^2+5\ge0\forall x\)

Vậy `2x^2 -4x+7` luôn dương với mọi `x`

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`-2x^2 - 16x + 38`

`<=> -2.(x^2+8x-19)`

Mà :\(x^2+8x-19\ge0\forall x\)

`=>` \(-2x.\left(x^2+8x-19\right)\le0\forall x\)

Vậy `-2x^2-16x+38` luôn âm với mọi `x`

\(\left(x+2\right)^2=x^2+4x+4\)  

Áp dụng hằng đẳng thức 

\(\left(x+y\right)^2=x^2+2xy+y^2\)

`(x+2)^2 = x^2 + 2.2.x + 2^2 = x^2 + 4x + 4`

Áp dụng HĐT số `1` 

`(x+y)^2 = x^2 + 2.x.y + y^2`

a) Điều kiện \(x\ne0\). Ta có 

\(A=\dfrac{x^2-x+2}{x^2}=1-\dfrac{1}{x}+\dfrac{2}{x^2}\)

Đặt \(\dfrac{1}{x}=p\) , khi đó \(A=2p^2-p+1\) 

Lại có \(A=2p^2-p+1=2\left(p^2-\dfrac{p}{2}+\dfrac{1}{2}\right)\) \(=2\left(p^2-2p.\dfrac{1}{4}+\dfrac{1}{16}+\dfrac{7}{16}\right)=2\left[\left(p-\dfrac{1}{4}\right)^2+\dfrac{7}{16}\right]\) \(=2\left(p-\dfrac{1}{4}\right)^2+\dfrac{7}{8}\)

Mà \(2\left(p-\dfrac{1}{4}\right)^2\ge0\Leftrightarrow2\left(p-\dfrac{1}{4}\right)^2+\dfrac{7}{8}\ge\dfrac{7}{8}\) hay \(min_A=\dfrac{7}{8}\)

Dấu "=" xảy ra khi \(p-\dfrac{1}{4}=0\Leftrightarrow p=\dfrac{1}{4}\Leftrightarrow\dfrac{1}{x}=\dfrac{1}{4}\Leftrightarrow x=4\)

Vậy GTNN của A là \(\dfrac{7}{8}\) khi \(x=4\)

b) \(B=\dfrac{x^2-2x+3}{\left(x+1\right)^2}=\dfrac{\left(x+1\right)^2-4x+2}{\left(x+1\right)^2}=1-\dfrac{4x+2}{\left(x+1\right)^2}\) \(=1-\dfrac{4\left(x+1\right)-2}{\left(x+1\right)^2}=1-\dfrac{4}{x+1}+\dfrac{2}{\left(x+1\right)^2}\)

Đến đây đặt \(\dfrac{1}{x+1}=t\) thì \(B=2t^2-4t+1\) và làm tương tự như câu a.

\(\dfrac{1}{4}x^2-\dfrac{1}{3}xy+\dfrac{1}{9}y^2=\left(\dfrac{1}{2}x\right)^2-2.\left(\dfrac{1}{2}x\right).\left(\dfrac{1}{3}y\right)+\left(\dfrac{1}{3}y\right)^2=\left(\dfrac{1}{2}x-\dfrac{1}{3}y\right)^2\)

\(4-6x+\dfrac{9}{4}x^2=2^2-2.2.\dfrac{2}{3}x+\left(\dfrac{2}{3}x\right)^2=\left(2-\dfrac{2}{3}x\right)^2\)

\(x^3+3x^2+3x+1=\left(x+1\right)^3\)

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

a, \(8-12x+6x^2-x^3=\left(2-x\right)^3\)

b, \(\dfrac{1}{4}x^2-\dfrac{1}{3}xy+\dfrac{1}{9}y^2=\left(\dfrac{1}{2}x-\dfrac{1}{3}y\right)^2\)

d, \(x^3+3x^2+3x+1=\left(x+1\right)^3\)

e, \(x^3\left(x^3+3x^2+3x+1\right)=x^3\left(x+1\right)^3\)

*Ta có: \(A=15x-23y=\left(13x-26y\right)+2x+3y⋮13\)

Mà \(13x-26y⋮13\)

\(\Rightarrow2x+3y⋮13\)

\(\Rightarrow B⋮13\)

* Ta có: \(A=15x-23y=\left(13x-26y\right)+2x+3y\)

Vì \(13x-26y⋮13\) và \(B=2x+3y⋮13\)

\(\Rightarrow A⋮13\)

đăng tách ra bạn nhé 

1, \(=\left(x+y-5\right)\left(x+y+5\right)\)

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

3, \(=\left(8x-8a-b\right)\left(8x+8a+b\right)\)

conect với tớ, tớ lưu lại bài trước khi nó mất

Giups em đi a

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

b, bạn ktra lại đề 

d, \(\left(3x-3y\right)^2-\left(2x+2y\right)^2=\left(3x-3y-2x-2y\right)\left(3x-3y+2x+2y\right)=\left(x-5y\right)\left(5x-y\right)\)

e, \(\left(2x-1\right)^2-\left(x+1\right)^2=\left(2x-1-x-1\right)\left(2x-1+x+1\right)=3x\left(x-2\right)\)

f,= \(\left(x+3\right)\left(x^2-3x+9\right)\)

g, \(=27x^3-\dfrac{1}{1000}=\left(3x-\dfrac{1}{10}\right)\left(9x^2+\dfrac{3}{10}x+\dfrac{1}{100}\right)\)

h, \(=\left(5x-1\right)\left(25x^2+10x+1\right)\)