(x+y)的平方=x^2+2xy+y^2
（x-y)的平方=x^2-2xy+y^2
(x+y)^2（x-y)^2
=(x^2-y^2)^2
=(x^2+2xy+y^2)(x^2-2xy+y^2)
=x^4-2x^3y+x^2y^2+2x^3y-4x^2y^2+2xy^3+x^2y^2-2xy^3+y^4
=x^4+x^2y^2-4x^2y^2+x^2y^2+y^4
=x^4-2x^2y^2+y^4
(x+y)^2*(x+y)^2
=x^2-y^2+2xy-2xy
=x^2-y^2
(x+y)^2=x^2+2xy+y^2
(x-y)^2=x^2-2xy+y^2
(x+y)的平方=(x+y)(x+y)
（x-y)的平方=(x-y)(x-y)
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