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1 sai
(a-b).(a+b)=a^2-b^2
2 đúng
3 đúng
4 sai
(x-3)^2=-(3-x)^2
5 sai
(x-3)^3=-(3-x)^3
![](https://rs.olm.vn/images/avt/0.png?1311)
xy + 2x + y + 2 = y(x + 1) + 2(x + 1) = (x + 1).(y + 2)
x(x - 1) + x(x + 3) = x(x - 1 + x + 3) = x. ( 2x + 2) = 2x.(x + 1)
\(-4x^2+8x-4=-4\left(x^2-2x+1\right)=-4\left(x-1\right)^2\)
c: \(-4x^2+8x-4\)
\(=-4\left(x^2-2x+1\right)\)
\(=-4\left(x-1\right)^2\)
![](https://rs.olm.vn/images/avt/0.png?1311)
a: Ta có: \(\left(x+3\right)\left(x+4\right)\left(x+5\right)\left(x+6\right)+1\)
\(=\left(x^2+9x+18\right)\left(x^2+9x+20\right)+1\)
\(=\left(x^2+9x\right)^2+38\left(x^2+9x\right)+360+1\)
\(=\left(x^2+9x\right)^2+2\cdot\left(x^2+9x\right)\cdot19+19^2\)
\(=\left(x^2+9x+19\right)^2\)
b. \(x^2+y^2+2x+2y+2\left(x+1\right)\left(y+1\right)+2\)
\(=\left(x^2+2x+1\right)+2\left(x+1\right)\left(y+1\right)+\left(y^2+2y+1\right)\)
\(=\left(x+1\right)^2+2\left(x+1\right)\left(y+1\right)+\left(y+1\right)^2\)
\(=\left(x+1+y+1\right)^2=\left(x+y+2\right)^2\)
c. \(x^2-2x\left(y+2\right)+y^2+4y+4\)
\(=x^2-2x\left(y+2\right)+\left(y+2\right)^2\)
\(=\left(x-y-2\right)^2\)
d. \(x^2+2x\left(y+1\right)+y^2+2y+1\)
\(=x^2+2x\left(y+1\right)+\left(y+1\right)^2\)
\(=\left(x+y+1\right)^2\)
![](https://rs.olm.vn/images/avt/0.png?1311)
\(x\left(x+1\right)=x^2+x\)