(x+y)的平方-(x-y)的平方 用平方差公式分解因式

解：(x+y)^2-(x-y)^2
={(x+y)+(x-y)}{(x+y)-(x-y)}
=2x * 2y
=4xy

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

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