x²-y²-3x-3y=x²-y²-3(x+y)
=(x+y)(x-y)-3(x+y)
=(x+y)[(x-y)-3]
=(x+y)(x-y-3)=(x+y)(x-y)-3(x+y) =(x+y)[(x-y)-3]为什么后面的（x+y）不见了提公因式：(x+y)(x-y)-3(x+y) =(x+y)[(x-y)-3]
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