Question

[Đặng Huỳnh Trâm]

Rút gọn biểu thức:

a) (x+y)^2+(x-y)^2+(x+y).(x-y)

b) (3x+y)^2+(x-3y)2-(2x+y).(2x-y)

c) 2(x-y).(x+y)+(x+y)^2+(x-y)^2

d)-2(x^2-9y^2)+(x-3y)^2+(x+3y)^2

Answer 1

[Trần Việt Linh]

a) \(\left(x+y\right)^2+\left(x-y\right)^2+\left(x+y\right)\left(x-y\right)\)

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

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

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

\(=9x^2+6xy+y^2+9x^2-6xy+y^2-4x^2+y^2\)

\(=14x^2+3y^2\)

c) \(2\left(x-y\right)\left(x+y\right)+\left(x+y\right)^2+\left(x-y\right)^2\)

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

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

\(=4x^2\)

d)\(-2\left(x^2-9y^2\right)+\left(x-3y\right)^2+\left(x+3y\right)^2\)

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

\(=\left(x+3y-x+3y\right)^2=9y^2\)