\(=\left(x-y\right)\left(x+y\right)\)

= ( x-y).(x+y)

( A   –   B ) 3   =   ( A   +   ( - B ) ) 3       =   A 3   +   3 . A 2 . ( - B )   +   3 . A . ( - B ) 2   +   ( - B ) 3       =   A 3   –   3 A 2 B   +   3 A B 2   –   B 3     = >   ( A   –   B ) 3   =   A 3   –   3 A 2 B   –   3 A B 2   +   B 3

là sai

Đáp án cần chọn là: B

`(x+y+z)(x+y+z)`

`=(x+y+x)^2`

`=(x+y)^2(x+z)^2(z+y)^2`

Lời giải:
$(x+y+z)(x+y+z)=(x+y+z)^2=x^2+y^2+z^2+2(xy+yz+xz)$

a) \((a+y)^3=a^3+3a^2y+3ay^2+y^3\)

b) \((x-b)^3=x^3-3x^2b+3xb^2-b^3\)

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

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

\(=6x^2y+2y^3\)

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

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

\(=8y^3+6y\left(x^2-y^2\right)\)

\(=8y^3+6x^2y-6y^3\)

\(=2y^3+6x^2y\)

(x+4)(x-4)

= x² - 4²

= x² - 16

Áp dụng HĐT: (A-B)(A+B)= A² - B²

(x+4).(x-4)

= x^2  -  4^2

=  x^2  -  16

`(x+4)(x-4)`

`=x^2 - 4^2`

`= x^2-16`

Vận dụng HĐT : `A^2 -B^2=(A-B)(A+B)`

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

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

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

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

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

Y = a^3 - b^3

Y = ( a - b ) ( a^2 + a . b + b^2 )

chỉ có thể là \(\left(a+b\right)^2-2ab\)thôi bạn ơi

T I C K nha 

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