Ta có: \(A=\left(x-y-1\right)^3-\left(x-y+1\right)^3+6\left(x-y\right)^2\)

\(=\left(x-y-1-x+y-1\right)\left[\left(x-y-1\right)^2+\left(x-y-1\right)\left(x-y+1\right)+\left(x-y+1\right)^2\right]+6\left(x-y\right)^2\)

\(=-2\cdot\left[3\left(x-y\right)^2+1\right]+6\left(x-y\right)^2\)

\(=-6\left(x-y\right)^2+6\left(x-y\right)^2-2\)

=-2

D  nhá!!

(-3*x-1)*y+3*x^2+x

D.(x - y)(3x + 1)

\(=x^2+2xy+y^2+x^2-2xy+y^2-2x^2=2y^2\)

\(=x^2+2xy+y^2+x^2-2xy+y^2-2x^2=2y^2\)

\(= x 2 + 2 x y + y 2 + x 2 − 2 x y + y 2 − 2 x 2 = 2 y 2\)

A

lộn

4*x*y câu C bạn nhé 

Ta có:

x x - y - y y - x = x 2 - x y - y 2 - x y = x 2 - x y - y 2 + x y = x 2 - y 2

Chọn (B) x 2 - y 2

Chọn đáp án A.

\(=\dfrac{1}{y-x}\cdot x^3\cdot\left(x-y\right)=-x^3\)

\(2,=x^2-3^2=\left(x-3\right)\left(x+3\right)\\ 3,=\left(x+y-x+y\right)\left(x+y+x-y\right)\\ =2y\cdot2x=4xy\)

x^2-9=x^2-3^2=(x+3)(x-3)
(x+y)^2-(x-y)^2=(x+y+x-y)(x+y-x-y)=2x

x²-3²=(x-3)(x+3)

(x+y)²-(x-y)²=(x+y-x+y)(x+y+x-y)=2y⋅2x=4xy

\(A=\frac{y}{x}\cdot\sqrt{\frac{x^2}{y^4}}=\frac{y}{x}\cdot\frac{\sqrt{x^2}}{\sqrt{y^4}}=\frac{y}{x}\cdot\frac{\left|x\right|}{\left|y^2\right|}=\frac{y}{x}\cdot\frac{x}{y^2}=\frac{1}{y}\)( x > 0 ; y > 0 )

\(A=\frac{x}{y}.\frac{x}{y^2}=\frac{x^2}{y^3}\left(\text{vì }x>0;y< 0\text{ nên: }\frac{x}{y^2}>0\right)\)

\(A=\frac{x}{y}\cdot\sqrt{\frac{x^2}{y^4}}=\frac{x}{y}\cdot\frac{\sqrt{x^2}}{\sqrt{y^4}}=\frac{x}{y}\cdot\frac{\left|x\right|}{\left|y^2\right|}=\frac{x}{y}\cdot\frac{x}{y^2}=\frac{x^2}{y^3}\)( x > 0 ; y < 0 )