2(x^2+y^2)^2-8xy(x^2+y^2)+8x^2y^2

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2(x^2+y^2)^2-8xy(x^2+y^2)+8x^2y^2

2(x^2+y^2)^2-8xy(x^2+y^2)+8x^2y^2
2(x^2+y^2)^2-8xy(x^2+y^2)+8x^2y^2

2(x^2+y^2)^2-8xy(x^2+y^2)+8x^2y^2
2(x^2+y^2)^2-8xy(x^2+y^2)+8x^2y^2
=2[(x^2+y^2)^2-4xy(x^2+y^2)+(2xy)^2]
=2[x^2+y^2-2xy]^2
=2(x-y)^4

2(x^2+y^2)^2-8xy(x^2+y^2)+8x^2y^2
设x^2+y^2=a,xy=b
所以
2a^2-8ab+8b^2
=2(a^2-4ab+4b^2)
=2(a-2b)^2
=2(x^2+y^2-2xy)^2
=2{(x-y)^2}^2
=2(x-y)^4

2(x^2+y^2)^2-8xy(x^2+y^2)+8x^2y^2
=2[(x^2+y^2)^2-4xy(x^2+y^2)+(2xy)^2]
=2[x^2+y^2-2xy]^2
=2(x-y)^4