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

`=[(x-y)-(x+y)]^2`

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

`=(-2y)^2`

`=4y^2`

Vậy `(x-y)^2+(x+y)^2-2(x+y)(x-y)=4y^2`

____

`***` áp dụng hằng đẳng thức:

`(a-b)^2=a^2-2ab+b^2`

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