\(\left(x+y-1\right)\left(x+y+1\right)=x^2+xy-x+xy+y^2-y+x+y-1\\ =x^2+\left(xy+xy\right)+\left(-x+x\right)+y^2+\left(-y+y\right)-1\\ =x^2+2xy+y^2-1\\ =>B\)

a.

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

= x^2 + y^2 - 2xy + x^2 + y^2 + 2xy

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

= 2x^2 + 2y^2

b.

(x^2 + y^2 - 2xy) - (x^2 + y^2 + 2xy)

= x^2 + y^2 - 2xy - x^2 - y^2 - 2xy

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

= -4xy

giúp tui với mấy bạn ơi

A

Uả bạn đang kiểm tra hay sao mà gấp thế?

\(\left(x^3+3x^2y+3xy^2+y^3-z^3\right):\left(x+y-z\right)\\ =\left[\left(x+y\right)^3-z^3\right]:\left(x+y-z\right)\\ =\left(x+y-z\right)\left[\left(x+y\right)^2+z\left(x+y\right)+z^2\right]:\left(x+y-z\right)\\ =x^2+2xy+y^2+xz+yz+z^2\)

Vậy chọn A 

\(\dfrac{x}{x^2+2xy+y^2}+\dfrac{2y}{x+y}+\dfrac{y}{x^2+2xy+y^2}\)

\(=\dfrac{x+y}{\left(x+y\right)^2}+\dfrac{2y}{x+y}\)

\(=\dfrac{1}{x+y}+\dfrac{2y}{x+y}=\dfrac{2y+1}{x+y}\)

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

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

\(\left(x-z\right)\left(x+z\right)=x^2+xz-xz-z^2=x^2-z^2\)

DÀI THẾ AI LÀM NỔI

x 2 +y 2 xy ​ = 8 5 ​ ⇒x 2 +y 2 = 5 8xy ​ \Rightarrow P=\frac{\frac{8xy}{5}-2xy}{\frac{8xy}{5}+2xy}=\frac{8xy-10xy}{8xy+10xy}=\frac{-2}{18}=-\frac{1}{9}⇒P= 5 8xy ​ +2xy 5 8xy ​ −2xy ​ = 8xy+10xy 8xy−10xy ​ = 18 −2 ​ =− 9 1 ​