(a十b)^2一(a一b)^2
=a^2+2ab+b^2-(a^2-2ab+b^2)
=a^2+2ab+b^2-a^2+2ab-b^2
=a^2-a^2+(2ab+2ab)+(b^2-b^2)
=4ab
(x十y十z)^2
(x+y)^2+z^2+2(x+y)z
=x^2+y^2+2xy+z^2+2xz+2yz
=x^2+y^2+z^2+2xy+2xz+2yz
(一x十2y)^2
=(-x)^2+2(-x)(2y)+4y^2
=x^2-4xy+4y^2
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