因式分解：①x²+y²；②x²-y²；③-x²+y²；④-x²-y²

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因式分解：①x²+y²；②x²-y²；③-x²+y²；④-x²-y²
因式分解：①x²+y²；②x²-y²；③-x²+y²；④-x²-y²

因式分解：①x²+y²；②x²-y²；③-x²+y²；④-x²-y²
①x²+y²
②x²-y²=(x+y)(x-y)
③-x²+y²=y²-x²=(y+x)(y-x)
④-x²-y²=-(x²+y²)