\(A=2\left(x^3-y^3\right)-3\left(x+y\right)^2\)

\(A=2\left[\left(x-y\right)^3+3xy\left(x-y\right)\right]-3\left[\left(x-y\right)^2+4xy\right]\)

\(A=2\left[2^3+3xy.2\right]-3\left[2^2+4xy\right]\)

\(A=2\left[28+6xy\right]-3\left[4+4xy\right]\)

\(A=56+12xy-12-12xy=56-12=44\)

Ta có : \(3\left(x^2+y^2\right)-\left(x^3+y^3\right)\)

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

\(=3\left(x+y\right)^2-6xy-2\left(x^2+2xy+y^2-3xy\right)\)

\(=3\left(x+y\right)^2-6xy-2\left(x+y\right)^2+6xy\)

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

\(=2^2=4\)

Ta có:
\(3\left(x^2+y^2\right)-\left(x^3+y^3\right)+1\)
\(=3\left(x^2+y^2\right)-\left(x+y\right)\left(x^2+y^2-xy\right)+1\)    ( 1 )
Do x + y = 2 nên biểu thức ( 1 ) trở thành:
\(=3\left(x^2+y^2\right)-2\left(x^2+y^2-xy\right)+1\)
\(=3\left(x^2+y^2\right)-2\left(x^2+y^2\right)+2xy+1\)
\(=\left(x^2+y^2\right)+2xy+1\)
\(=\left(x+y\right)^2+1\)    ( 2 )
Do x + y = 2 nên biểu thức ( 2 ) trở thành:
\(=2^2+1=5\)
Vậy với x + y = 2 thì \(3\left(x^2+y^2\right)-\left(x^3+y^3\right)+1=5\)

Từ dòng thứ 3 mình thiếu cộng 1 

Bạn thêm vô là 5 nhé 

\(x+y+1=0\\ \Leftrightarrow x+y=-1\)

Thay x+y=-1 vào C ta có:

\(C=x^2\left(x+y\right)-y^2\left(x+y\right)+x^2-y^2+2\left(x+y\right)+3\)

\(\Rightarrow C=x^2\left(-1\right)-y^2\left(-1\right)+x^2-y^2+2\left(-1\right)+3\)

\(\Rightarrow C=-x^2+y^2+x^2-y^2-2+3\)

\(\Rightarrow C=\left(-x^2+x^2\right)+\left(y^2-y^2\right)+\left(3-2\right)\)

\(\Rightarrow C=0+0+1\)

\(\Rightarrow C=1\)

\(x+y+1=0\) =>\(x+y=-1\)

- Thay \(x+y=-1\) vào C ta được:

\(C=x^2\left(x+y\right)-y^2\left(x+y\right)+x^2-y^2+2\left(x+y\right)+3\)

\(=-x^2+y^2+x^2-y^2-2+3\)=1

Sao bạn doanh doanh nhắn chữ "hhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhh" quài vậy ?

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

\(x^3+y^3=\left(x+y\right)^3-3xy\left(x+y\right)=1-3xy\)

\(B=3\left(x^2+y^2\right)-2\left(x^3+y^3\right)\)

\(=3\left(1-2xy\right)-2\left(1-3xy\right)\)

\(=3-6xy-2+6xy\)

\(=1\)

\(3,x=\dfrac{1}{2},y=-1\)

\(\Rightarrow C=\dfrac{1}{2}\left[\left(\dfrac{1}{2}\right)^2+1\right]-\left(\dfrac{1}{2}\right)^2\left(\dfrac{1}{2}-1\right)-1\left[\left(\dfrac{1}{2}\right)^2-\dfrac{1}{2}\right]\)

\(\Rightarrow C=\dfrac{1}{2}\left(\dfrac{1}{4}+1\right)-\dfrac{1}{4}\left(-\dfrac{1}{2}\right)-\left(\dfrac{1}{4}-\dfrac{1}{2}\right)\)

\(\Rightarrow C=\dfrac{1}{2}.\dfrac{5}{4}+\dfrac{1}{8}-\left(-\dfrac{1}{4}\right)\)

\(\Rightarrow C=\dfrac{5}{8}+\dfrac{1}{8}+\dfrac{1}{4}\)

\(\Rightarrow C=1\)

\(4,x=\dfrac{1}{2},y=-100\)

\(\Rightarrow D=\dfrac{1}{2}\left[\left(\dfrac{1}{2}\right)^2+100\right]-\left(\dfrac{1}{2}\right)^2\left(\dfrac{1}{2}-100\right)-100\left[\left(\dfrac{1}{2}\right)^2-\dfrac{1}{2}\right]\)

\(\Rightarrow D=\dfrac{1}{2}\left(\dfrac{1}{4}+100\right)-\dfrac{1}{4}\left(-\dfrac{199}{2}\right)-100\left(\dfrac{1}{4}-\dfrac{1}{2}\right)\)

\(\Rightarrow D=\dfrac{1}{2}.\dfrac{401}{4}+\dfrac{199}{8}-100.\left(-\dfrac{1}{4}\right)\)

\(\Rightarrow D=\dfrac{401}{8}+\dfrac{199}{8}+25\)

\(\Rightarrow D=100\)

3: C=x^3-xy-x^3-x^2y+x^2y-xy

=-2xy=-2*1/2*(-1)=1

4: D=x^3-xy-x^3-x^2y+x^2y-xy

=-2xy

=-2*1/2*(-100)=100

\(x+y=1\)

\(\Leftrightarrow\)\(\left(x+y\right)^2=1\)

\(\Leftrightarrow\)\(x^2+y^2=1-2xy\)

\(x+y=1\)

\(\Leftrightarrow\)\(\left(x+y\right)^3=1\)

\(\Leftrightarrow\)\(x^3+y^3=1-3xy\)

\(H=1-3xy+3xy\left(1-2xy\right)+6x^2y^2\left(xy+y\right)\)

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

\(=1-6x^2y^2\left(1-xy-y\right)\)

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

\(=1-6x^2y^2\left(x-xy\right)\)

\(=1-6x^3y^2\left(1-y\right)\)

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

\(=1-6x^4y^2\)

mới ra đc đến đây

\(A=3\left(x^2+y^2\right)-2\left(x^3+y^3\right)\)

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

\(=3x^2+3y^2-2.1\left(x^2-xy+y^2\right)\)

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

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

\(B=x^3+y^3+3xy\left(x^2+y^2\right)+6x^2y^2\left(x+y\right)\)

\(=x^3+y^3+3xy\left[\left(x+y\right)^2-2xy\right]+6x^2y^2.1\)

\(=x^3+y^3+3xy\left(x+y\right)^2-6x^2y^2+6x^2y^2\)

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

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

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