( 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

\(7)\)  \(\left(3x\right)^2-y^2=\left(3x-y\right)\left(3x+y\right)\)

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

\(9)\)  \(\left(2x\right)^2-y^2=\left(2x-y\right)\left(2x+y\right)\)

\(=x^3+9x^2+27x+27\)

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

(1𝑦/3+3)^3

(𝑦/3+3)^3

(𝑦/3+3⋅3/3)^3

(𝑦+3⋅3/3)^3

(𝑦+9/3)^3

\(\left(\dfrac{1}{3}y+3\right)^3=\dfrac{1}{27}y^3+y^2+9y+27\)

\(\left(x-6\right)\left(6+x\right)=\left(x-6\right)\left(x+6\right)=x^2-6^2=x^2-36\)

=(x-6) (x+6= \(x^2-6^2=x^2-36\)

\(\left(3x^2+\dfrac{2}{3}y\right)^2\)

\(=\left(3x^2\right)^2+2.3x.\dfrac{2}{3}y+\left(\dfrac{2}{3}y\right)^2\)

\(=9x^4+4xy+\dfrac{4}{9}y^2\)

(3x²+\(\dfrac{2}{3}\)y)²

=9\(x^4\)+\(\dfrac{4}{9}\)\(y^2\)

a: \(\left(6x^2+\frac13\right)^2\)

\(=\left(6x^2\right)^2+2\cdot6x^2\cdot\frac13+\left(\frac13\right)^2\)

\(=36x^4+4x^2+\frac19\)

b: \(\left(5x-4y\right)^2=\left(5x\right)^2-2\cdot5x\cdot4y+\left(4y\right)^2\)

\(=25x^2-40xy+16y^2\)

a: \(=x^2-12x+36\)

b: \(=x^3+9x^2+27x+27\)

A.x=6

b.x=-3