`A= x^2+2xy-3x^2 +2y^2+3x^2-y^2`

`= (x^2-3x^2 +3x^2) +2xy +(2y^2 -y^2)`

`= x^2 +2xy +y^2`

`=(x+y)^2`

A = \(x^2\) + 2\(xy\) - 3\(x^2\) + 2y² + 3\(x^2\) - y²

A = (\(x^2\)- 3\(x^2\) + 3\(x^2\)) + 2\(xy\) + (2\(y^2\) - y²)

A = \(x^2\) + 2\(xy\) + y²

A = (\(x\) + y)²

Lời giải:

PT $\Leftrightarrow 8x^3-16x^2+6x+2=0$

$\Leftrightarrow (8x^3-8x^2)-(8x^2-8x)-(2x-2)=0$

$\Leftrightarrow 8x^2(x-1)-8x(x-1)-2(x-1)=0$

$\Leftrightarrow (x-1)(8x^2-8x-2)=0$

$\Leftrightarrow 2(x-1)(4x^2-4x-1)=0$

$\Leftrightarrow x-1=0$ hoặc $4x^2-4x-1=0$

Nếu $x-1=0\Leftrightarrow x=1$ 

Nếu $4x^2-4x-1=0$

$\Leftrightarrow (2x-1)^2-2=0$

$\Leftrightarrow (2x-1-\sqrt{2})(2x-1+\sqrt{2})=0$

$\Leftrightarrow x=\frac{1\pm \sqrt{2}}{2}$

\((x+3y)^2\\=x^2+2\cdot x\cdot3y+(3y)^2\\=x^2+6xy+9y^2\\---\\(x-5xy)^2\\=x^2-2\cdot x\cdot5xy+(5xy)^2\\=x^2-10x^2y+25x^2y^2\)

\((5+9y)^3\\=5^3+3\cdot5^2\cdot9y+3\cdot5\cdot(9y)^2+(9y)^3\\=125+675y+1215y^2+729y^3\\---\\(6x-7xy)^3\\=(6x)^3-3\cdot(6x)^2\cdot7xy+3\cdot6x\cdot(7xy)^2-(7xy)^3\\=216x^3-756x^3y+882x^3y^2-343x^3y^3\)

\(4\left(x+3\right)\left(x^2-3x+9\right)-x\left(4x^2-3x\right)=6x+108\)

\(\Leftrightarrow4\left(x^3+27\right)-\left(4x^3-3x^2\right)=6x+108\)

\(\Leftrightarrow4x^3+108-4x^3+3x^2=6x+108\)

\(\Leftrightarrow3x^2+108=6x+108\)

\(\Leftrightarrow3x^2-6x=108-108\)

\(\Leftrightarrow3x\left(x-2\right)=0\)

\(\Leftrightarrow\left[{}\begin{matrix}3x=0\\x-2=0\end{matrix}\right.\)

\(\Leftrightarrow\left[{}\begin{matrix}x=0\\x=2\end{matrix}\right.\)

Do ABCD là hình thang cân (AB//CD) nên:

\(\widehat{A}+\widehat{D}=180^o\)

Mà: \(\widehat{C}=\widehat{D}\)

\(\Rightarrow\widehat{A}+\widehat{C}=180^o\)

\(\Rightarrow\widehat{A}=\left(180^o+20^o\right):2=100^o\)

\(\Rightarrow\widehat{C}=180^o-100^o=80^o\)  

Vậy số đo của góc \(\widehat{A}\) và \(\widehat{C}\) lần lượt là: \(100^o\) và \(80^o\)

⇒ Chọn B 

a. $x^2+4x+4$

$=x^2+2\cdot x\cdot2+2^2$

$=(x+2)^2$

b. $x^2-6xy+9y^2$

$=x^2-2\cdot x\cdot3y+(3y)^2$

$=(x-3y)^2$

c. $4x^2+12x+9$

$=(2x)^2+2\cdot2x\cdot3+3^2$

$=(2x+3)^2$

d. $x^2-x+\dfrac14$

$=x^2-2\cdot x\cdot \dfrac12+\Bigg(\dfrac12\Bigg)^2$

$=\Bigg(x-\dfrac12\Bigg)^2$

`x^2 +4x+4`

`=x^2+2*x*2+2^2`

`=(x+2)^2`

__

`x^2-6xy+9y^2`

`=x^2 - 2*x*3y+(3y)^2`

`=(x-3y)^2`

__

`4x^2 +12x+9`

`=(2x)^2 +2*2x*3+3^2`

`=(2x+3)^2`

__

`x^2-x+1/4`

`=x^2 - 2*x*1/2 +(1/2)^2`

`=(x+1/2)^2`

ok man

hình đâu ?:>

WHERE?